* Revert "Roll dart to 4bcf8ad411bd90da16d44ad399bf350e8423eac9 (#4929)"

Fixes https://github.com/flutter/flutter/issues/16149

This is to support Swift users, where nil is allowed for CVPixelBufferRef.

Fixes https://github.com/flutter/flutter/issues/16055

Includes:
* Derive iOS host toolchain from $host_cpu (flutter/buildroot#131)

This is required so that Dart initialization can find the platform kernel
assets when running in Dart 2 mode

Fixes https://github.com/flutter/flutter/issues/16143

https://skia.googlesource.com/skia.git/+log/b33feaca1caa..9874bf1bcecd

$ git log b33feaca1..9874bf1bc --date=short --no-merges --format='%ad %ae %s'
2018-04-02 angle-skia-autoroll Roll skia/third_party/externals/angle2/ d104918fd..5a7e61bbd (1 commit)
2018-03-29 halcanary SkPDF/SkTypefaceMetrics: Set PostScript Name in Directwrite backend.
2018-04-02 djsollen Add multilib support as the default build option for libskqp
2018-04-02 mtklein In older STLs, std::vector::insert() returns void.
2018-04-02 benjaminwagner Revert "Temporarily remove Win10 QuadroP400 from CQ."
2018-04-02 halcanary SkPDF/Clusterator: a second test
2018-03-28 csmartdalton ccpr: Normalize homogeneous coords with frexp/ldexp
2018-04-02 robertphillips Turn on rendertarget sorting
2018-03-22 mtklein implement SkTDArray with std::vector
2018-04-02 egdaniel Remove specVersion check on vk extensions.
2018-04-02 kjlubick Add mock gl fuzzer
2018-04-02 skcms-skia-autoroll Roll skia/third_party/externals/skcms/ a0790e0c9..d2cc6a7f7 (1 commit)
2018-04-02 kjlubick Roll Pixel 2 XL version
2018-03-21 caryclark fix SkRRect fuzzer assert
2018-04-02 kjlubick More nans causing infinite recursion in GrPathUtils::convertCubicToQuads
2018-04-02 caryclark fix a couple of minor surface doc changes
2018-03-29 ethannicholas fix for frexp dataflow analysis
2018-04-01 update-skps Update SKP version
2018-03-31 rmistry Revert "Whitespace change"

Created with:
  roll-dep ../src/third_party/skia

When we converted this code to FIDL2, we moved the FlatNamespace into
SetupNamespace, which meant it grabed the service root handle instead of
leaving it in-place for ApplicationContext::CreateFrom to pick it up.
Now, we pass this object by pointer, which is what we did in FIDL1.

This API is being hidden for safety reasons. Instead, use a locally
defined rect that's slightly smaller (so operations don't overflow).

https://skia.googlesource.com/skia.git/+log/abf7b763e2c1..b33feaca1caa

$ git log abf7b763e..b33feaca1 --date=short --no-merges --format='%ad %ae %s'
2018-03-31 update-docs Update markdown files
2018-03-31 angle-skia-autoroll Roll skia/third_party/externals/angle2/ b33972d4a..d104918fd (1 commit)
2018-03-30 kjlubick Add Encoder fuzzers
2018-03-30 robertphillips Add SkSurface factory that takes an SkSurfaceCharacterization
2018-03-30 skcms-skia-autoroll Roll skia/third_party/externals/skcms/ a8bceef13..a0790e0c9 (2 commits)
2018-03-30 csmartdalton Revert "Delete GPU alpha clip masks"
2018-03-30 mtklein Revert "allow timing in release-with-assert builds"
2018-03-30 angle-skia-autoroll Roll skia/third_party/externals/angle2/ b1de5a7e6..b33972d4a (1 commit)
2018-03-30 mtklein allow timing in release-with-assert builds
2018-03-30 phosek Disable GPU support by default when building for host in Fuchsia
2018-03-30 skcms-skia-autoroll Roll skia/third_party/externals/skcms/ d8ece14aa..a8bceef13 (1 commit)
2018-03-30 angle-skia-autoroll Roll skia/third_party/externals/angle2/ 5cf4d06cd..b1de5a7e6 (2 commits)
2018-03-29 csmartdalton Revert "vulkan: Blacklist MSAA path renderer on Qualcomm drivers"
2018-03-29 angle-skia-autoroll Roll skia/third_party/externals/angle2/ 96310cdad..5cf4d06cd (5 commits)
2018-03-29 robertphillips Add SK_API to GrContextThreadSafeProxy
2018-03-29 skcms-skia-autoroll Roll skia/third_party/externals/skcms/ ffeadc2cb..d8ece14aa (1 commit)
2018-03-28 halcanary SkPDF/Clusterator: expose to the light of a unit test
2018-03-29 skcms-skia-autoroll Roll skia/third_party/externals/skcms/ 11231dcb6..ffeadc2cb (1 commit)
2018-03-29 metzman Add libFuzzer style fuzzer for NullGLCanvas for use on OSS-Fuzz.
2018-03-29 robertphillips Fix up .fp files
2018-03-29 csmartdalton vulkan: Blacklist MSAA path renderer on Qualcomm drivers
2018-03-29 skcms-skia-autoroll Roll skia/third_party/externals/skcms/ 193fc66b4..11231dcb6 (3 commits)
2018-03-29 halcanary SkAdvancedTypefaceMetrics:  distinguish between font names
2018-03-29 skcms-skia-autoroll Roll skia/third_party/externals/skcms/ 115e2853e..193fc66b4 (1 commit)
2018-03-29 jvanverth Snap non-scaled emoji to pixel borders
2018-03-29 djsollen Dump additional details about the gpu resources.
2018-03-29 halcanary SkPDF: type3: more descriptor values
2018-03-29 skcms-skia-autoroll Roll skia/third_party/externals/skcms/ 3f0009288..115e2853e (2 commits)
2018-03-29 bungeman Remove path ranges from gpu.
2018-03-28 halcanary SkDynamicMemoryWStream: clean up, asserts, etc.
2018-03-28 senorblanco Reland "GrTessellator: hang fix."
2018-03-29 angle-skia-autoroll Roll skia/third_party/externals/angle2/ a3b220f36..96310cdad (1 commit)
2018-03-29 skcms-skia-autoroll Roll skia/third_party/externals/skcms/ 5678b9d6a..3f0009288 (1 commit)
2018-03-28 kdonev Fixing build with SK_IGNORE_TO_STRING defined.
2018-03-28 ethannicholas fixed type mismatches in SkSL comparison operators
2018-03-28 csmartdalton Don't use gl_FragCoord on legacy Tegra hardware
2018-03-29 mtklein Revert "GrTessellator: hang fix."
2018-03-29 angle-skia-autoroll Roll skia/third_party/externals/angle2/ 78c5eb2ab..a3b220f36 (7 commits)
2018-03-28 skcms-skia-autoroll Roll skia/third_party/externals/skcms/ ed6a9bd48..5678b9d6a (1 commit)
2018-03-28 liyuqian Update trans limit after zoom changes in viewer
2018-03-14 rmistry Whitespace change
2018-02-15 bungeman Add color fonts.
2018-03-28 skcms-skia-autoroll Roll skia/third_party/externals/skcms/ 59fdb4e14..ed6a9bd48 (2 commits)
2018-03-28 senorblanco GrTessellator: hang fix.
2018-03-28 robertphillips Add new GrResourceCache::purgeUnlockedResources variant
2018-03-28 bungeman Check that paint is not null in OveridePaintFilterCanvas.
2018-03-27 robertphillips Remove old method of drawing DDL SKPs from DM (in favor of ViaDDL method)
2018-03-28 skcms-skia-autoroll Roll skia/third_party/externals/skcms/ 680ab5a31..59fdb4e14 (3 commits)
2018-03-28 angle-skia-autoroll Roll skia/third_party/externals/angle2/ c7575da54..78c5eb2ab (1 commit)
2018-03-28 skcms-skia-autoroll Roll skia/third_party/externals/skcms/ fc0d0c4c7..680ab5a31 (1 commit)
2018-03-27 bungeman Roll FreeType.
2018-03-27 halcanary SkPDF: Accept  non-monotonic text cluster indices
2018-03-27 csmartdalton Delete GPU alpha clip masks
2018-03-28 jvanverth Rename SkInsetConvexPolygon to SkOffsetPolygon.
2018-03-27 robertphillips Add handling for allocation failure in GrTextureOpList
2018-03-28 angle-skia-autoroll Roll skia/third_party/externals/angle2/ e858cb1db..c7575da54 (1 commit)
2018-03-27 skcms-skia-autoroll Roll skia/third_party/externals/skcms/ 80ac04dc4..fc0d0c4c7 (1 commit)
2018-03-27 angle-skia-autoroll Roll skia/third_party/externals/angle2/ 15c7d0a64..e858cb1db (12 commits)
2018-03-27 herb Various cleanup to remote_demo
2018-03-27 skcms-skia-autoroll Roll skia/third_party/externals/skcms/ 6793c6ab0..80ac04dc4 (2 commits)
2018-03-27 robertphillips Reland "Switch bots to use ViaDDL (i.e., ddl-gl) instead of old --ddl flag"
2018-03-27 skcms-skia-autoroll Roll skia/third_party/externals/skcms/ aa8c0f3d2..6793c6ab0 (1 commit)
2018-03-27 ethannicholas initial SkSLJIT checkin
2018-03-27 bungeman Revert "Add color fonts."
2018-03-27 mtklein exclude SkColorSpaceXform_skcms.cpp from Google3
2018-03-27 skcms-skia-autoroll Roll skia/third_party/externals/skcms/ 07da72a48..aa8c0f3d2 (1 commit)
2018-02-15 bungeman Add color fonts.
2018-03-27 bungeman Remove include/svg from public.bzl includes.
2018-03-27 skcms-skia-autoroll Roll skia/third_party/externals/skcms/ 7e5292bb2..07da72a48 (1 commit)
2018-03-27 brianosman Remove duplicate entry for SkTDPQueue.h in gni
2018-03-27 mtklein cache SkMatrix44 type in makeColorSpin()
2018-03-27 robertphillips Fix flushing of context in ViaDDL
2018-03-26 csmartdalton ccpr: Various cleanups
2018-03-27 skcms-skia-autoroll Roll skia/third_party/externals/skcms/ 286108ae1..7e5292bb2 (1 commit)
2018-03-27 brianosman Pin color type when a cross-context image is constructed
2018-03-26 benjaminwagner Update doc for connecting to GCE bots.
2018-03-27 robertphillips Fix TSAN race in SkPaintToGrPaintNoShader
2018-03-27 robertphillips Revert "Switch bots to use ViaDDL (i.e., ddl-gl) instead of old --ddl flag"
2018-03-27 angle-skia-autoroll Roll skia/third_party/externals/angle2/ 84fdc62c6..15c7d0a64 (1 commit)
2018-03-20 kdonev Fixed warnings for casting large type to smaller one.
2018-03-27 robertphillips Switch bots to use ViaDDL (i.e., ddl-gl) instead of old --ddl flag
2018-03-26 brianosman Add SkColorSpaceXform_skcms
2018-03-27 liyuqian Reland "Use DAA for small cubics and non-convex paths that fit into a mask"
2018-03-27 robertphillips Switch DDL rendering to be a Via in DM (take 2)
2018-03-23 herb Remove SkAutoGlyphCache
2018-03-27 angle-skia-autoroll Roll skia/third_party/externals/angle2/ a571f28d4..84fdc62c6 (6 commits)
2018-03-26 benjaminwagner [Bazel] Add missing include dir.
2018-03-26 skcms-skia-autoroll Roll skia/third_party/externals/skcms/ db50cadbb..286108ae1 (1 commit)
2018-03-21 csmartdalton ccpr: Use Manhattan distance for curve AA
2018-03-26 benjaminwagner External part of internal cl/190527962
2018-03-26 bungeman Remove src/xml/*.h from DM in public.bzl.
2018-03-21 herb Move cache difference code into remote cache files
2018-03-26 herb Cleanup SkAutoGlyphCache in SkXPSDevice.
2018-03-26 robertphillips Revert "Switch DDL rendering to be a Via in DM"
2018-03-26 robertphillips Switch DDL rendering to be a Via in DM
2018-03-21 csmartdalton ccpr: Replace curve corner MSAA with analytic attenuation
2018-03-26 bungeman Add the xml and svg model code to DM in public.bzl.
2018-03-26 ethannicholas added frexp support to SkSL
2018-03-26 robertphillips Remove copy of TableColorFilter data when GPU rendering
2018-03-26 mtklein specialize arm64 allTrue()/anyTrue()
(...)

Created with:
  roll-dep ../src/third_party/skia

This provides improved code inspection, making it easier for developers to figure out what might be going wrong if they override this and don't call super, like I just did. :-)

* Roll to 59d859645ef0557bc7f8e2ee07957f506b755255

Changes since last roll:
```
59d859645e [fasta] Do new/const insertion in simple cases
4c0a09b79f [vm-service] Add setFlag.
bed5debbeb Callable object support in Analyzer and DDC.
a1e4d8e0bf Don't do implicit new/const code generation with --preview-dart-2 enabled.
49558b9c4c Fixed MessageChannel receiveing messages on ports. Removed pkg browser from tests.
e150bf638c Fix tests for missing required SDK libraries on Windows.
b970d834e1 Update type_representation_test to test strong mode types
85d5812a93 Port type_representation_test to work with kernel
8504808ce3 Remove Dart 2 errors in package:js_ast
a6a2152482 Add runtime type representation for void
477a773cdb [fidl] Port Dart runtime to FIDL2.
d46dd1bd68 fix error message for dcalls on null
af22f6a591 Track calls to 'pub get' from analysis_server
8d689fcca7 step 4 of ??: add omit-implicit-checks and preview-dart-2 flags, strong-mode defaults to include implicit checks.
f201ace7f3 Check for missing required libraries dart:core and dart:async.
d53291d156 Fix MockErrorCode after adding isUnresolvedIdentifier.
b05ba71b0c [VM runtime] Minor clean up of type test with non-function type object.
ce1d52481c Don't remove unused imports when there are unresolved symbols.
ad9afa1656 Add the isUnresolvedIdentifier flag for ErrorCode.
14806df576 Make correction parameter optional named in ErrorCode and subclasses.
98f627f989 Issue 32708. Make GenericFunctionTypeElementForLink implement GenericFunctionTypeElementImpl.
3306aabd82 Fix for type inference from instance creation arguments.
055dfad887 Fix the test to determine whether a library is in the SDK
506ac7f953 Rework linker type inference using ExprBuilder.
494b861d00 Further prep work for #32525.
7d90df42f1 [Kernel, VM runtime] Hook up bool checks in strong mode (fixes #32336). Update kernel status file.
```

* Update license hash. No changes to licenses.

* Roll to pick up SIGTTOU fix.

* Update license hash. No changes to licenses

This reverts commit ca3d13f2.

pub rolll causes https://github.com/dart-lang/pub/issues/1853.

* Roll dart.

Changes since last roll:

4da52281aa [frontend-server] Return non-zero exit code if there are errors.
d7a540bfe7 Document --enable-asserts in dart2js's help, and remove --checked.
d9c3190e44 [vm] Remove some bin -> vm includes.
3dcdaf9e57 [vm] Check for OOM when allocating a new handle block.
865f52737a [dart:io] Adds IOOverrides.socketConnect
f858e5d539 Replace Token.previous usage in BodyBuilder
f66532b31b Replace more occurrences of token.previous with findPrevious
d46915d239 Bring in the latest pub
7470f410dd Clean up documentation of command-line analyzer options
4b682341d9 [ VM / Dart 2 ] Marked tests that exercise functionality not supported in Dart 2 as SkipByDesign.
859632b439 Fixes for uninitialized fields of Flutter widgets create constructors in Flutter style.
c9e9c1d8e9 Remove dart:isolate runtime support in DDC
0482cf4da1 [fuchsia] Add rules to build gen_snapshot targeting Fuchsia
2de82410e0 Make isolate patch play nice with web workers.
be3baa0b8f Additional ExprBuilder implementation.
42581e4fa6 [VM] Added '--enable-asserts' to the top-level help message.
44dbba2b80 Use offset/length of the import prefix, or empty. Same for constructor names.
85b4c604c5 [VM runtime] Improve type error messages in Dart 2 (fixes #32564). Hide private class _Future and map it to core lib class Future. In Dart 2, show dynamic as type argument(s) of raw generic types.
ebdee7ee0f More prep work to allow the linker to re-use resolution logic.
91e6c8b019 Fix missing_return warning in expr_builder.dart
59de9710ea Complete Flutter's setState() as a whole statement, with a closure, and the caret inside the closure.
b82e5b4704 Replace code completion usage of token.previous with findPrevious
0e8457c8c8 Fix for building sequence of identifiers and several tests.
6a84638d7a Updated for proper failure.
577b168c39 Fix jsinterop bug: we incorrectly added null values for js-interop constructor calls.
8e02ff92db Added @category annotation for Web libraries and removed createCustomUpgrader.
c60199cf86 [vm/kernel/aot] Tree shaking based on results of TFA, take 2.
b217e21907 Mark a few more co19 failures to fix.
70f455351d [kernel] Add field for instantiate-to-bounds algorithm in TypeParameter.
bc75856e54 Update getClientRects, removed xtab, removed bogus entry in dom.json, expose ServiceWorker, removed query and queryAll from in dart:html, added constructor to MessageChannel and removed getCssCanvasContext.
f9e12f1116 step 3 of ??: ignore old options from old FE
c0fa4e2fe3 step 2 of ??: remove unnecessary pattern in -m option
51867f28a2 step 1 of ??: cleaning up options...

* Update license hash. No license changes.

* add Path.addCircle

* docs formatting

* revert path.cc/path.h; remove addCircle and update docs on addOval

* add missing file.

* Rolled Dart SDK to e3051175192c83fafa3636c0f16d804734bb285a

Fixes https://github.com/flutter/flutter/issues/15975

Old API is going to be hidden/removed soon

Breaks Linux and Mac build bots.

This reverts commit d9ec2de7.

Fixes https://github.com/flutter/flutter/issues/15884

* Support for decimal and signed numeric keyboard

* Comments

* CONTRIBUTING.md: explain the need for building host with non-host

* address comments

* locally-built