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Dart SDK itself does not use pub which forced us to manually maintain dependecy
overrides in the frontend server's pubspec.yaml. This is rather fragile and
easily breaks when dependencies in one of the "internal" packages change, but
the package is not published yet.

This change switches us to the same model of dependcies management that Dart SDK
itself is using.

Down side of this is that we can no longer do pub run test in the
frontend_server and have to run tests manually.

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Add back the core snapshot to runtime_unittests and embedder_unittests.

Fixes
1. https://github.com/flutter/flutter/issues/12148
2. most part of https://github.com/flutter/flutter/issues/17731 except some tiny AA diffs.

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This reverts commit e0d4c464.

Not all platforms are expected to handle all SemanticsEvents. Therefore, it is ok to just drop unsupported events on the floor.

Allows one to use a different core snapshot without a custom engine build by just packaging a different one in the APK.

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Flutter AOT builds can be done on Android using .so files (instead of
separate instruction/data snapshots) using the `--build-shared-library`
flag.

Running from .so files stopped working after the engine refactoring in
58e84c8b, which this CL restores.

Issue https://github.com/flutter/flutter/issues/17236

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* split `children` into two ordered lists: traversal and hit test

* address comments

* reduce node object byte size

* link to DebugSemanticsDumpOrder

This causes Skia to fall back to the ES2 shading language when the GL
driver doesn't clearly communicate that it supports the ES3 external
image extension. Doing so ensures that video playback (which relies on
external images) works on many more devices.

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* Roll Dart to version 97b6c2e09d6532eeb622d85f23109f3f3b6d2b61

This dart roll includes the following changes:

97b6c2e09d Don't give Source and ResourceProvider to CompletionRequestImpl.
04816270fa Add Forest support for creating parenthesized conditions
7a90ed9b88 Tests for completion with nested blocks, functions, and methods.
353ddbaea1 Add Forest support for while statements
8d0bda43f3 Fix codesize regression from Cleanup after removing MirrorsData
802c25aff9 Add tests to test InstanceCreationExpressionImpl.canBeConst
def6f6c86c Don't create linked groups when nothing was written.
0c2b349827 Blocks and variable declarations in Forest
5ecf69d90a [fasta] Use Object as defaultType if Object is the explicit bound
4bac17655d [kernel/vm] VM support for instantiate-to-bounds.
a4494a7fb2 Make `mixin` and `interface` built-in identifiers.
dc76aa34c7 Stop using a port to keep the isolate alive in async_helper.
a16563931c [VM] Mark 3 more tests as pass, crash
a5d4195b0b CHANGELOG entry for noSuchMethod change
d49af06ce0 Fix for searching references to named parameters in hierarchy.
c26435f18f [dart:io] Add length check assert to x509 code
6fee620564 Convert more IdentiferContext consts
a6ad152315 Fix constant-related deprecation warnings in analyzer packages
3e8ca8e0a6 Improve class declaration header recovery
ffd5a1914d Update top level function name identifier context
24f5ca0187 [fuchsia] Use Fuchsia prebuilts
1a2f317735 Update parse mixin to use computeType and add recovery tests
1d6619eba6 [infra] Upgrade checked in SDKs to 2.0.0-dev.55.0
76a30dd594 Initial implementation of 'execution.getSuggestions'.
510669a0b4 [VM] Address uncovered issues, mark test as passing
abb1ee98f7 Move library related operations to DartFileEditBuilderImpl. Initialize enclosing elements into fields lazily.
7d5ebb937b A few more Analyzer/CFE type argument tests
11006c3bac Reland "[VM] Use IR for code in [CatchEntryInstr]s to populate captured exception/stacktrace variables"
e7495e427c [dart:io] Adds X509Certificate.der and X509Certificate.pem
62ffedefdd Rewrite writing types to using existing writeXYZ() methods.
e6fc78c87b Revert "Reland "[VM] Use IR for code in [CatchEntryInstr]s to populate captured exception/stacktrace variables""
d59745029a [fasta] Update expectation files after CL 50945
6fc4854e98 [fasta] Run instantiate-to-bound on functions
23b4a505ec Reland "[VM] Use IR for code in [CatchEntryInstr]s to populate captured exception/stacktrace variables"
23c8c4d101 [vm/kernel] Pass type variables through ActivationFrame for expression compilation.
b827b70e89 Extract IdentifierContext.typeVariableContext into new subclass
b7659e6d03 Improve typedef declaration recovery
7c1ae73a21 Fix a Dart 2 runtime bug in BodyBuilder
38cb4cbc0a [VM] Update status file to make hot-reload builders green
1be81b4a96 [VM] Skip vm/dart/appjit_test on opt-counter builder
4d22c0de58 [infra] Improve cycle time of dart2js builders
812189d3e7 Convert LargeIntAccessor to LargeIntAccessGenerator
5e98c49606 Convert ReadOnlyAccessor to ReadOnlyAccessGenerator
17e284fbc2 Convert DeferredAccessor to DeferredAccessGenerator
d5719cae62 Small cleanup in compiler.dart
54173bf293 Move TypeMask cache to CommonMasks
78f348b3d5 Don't export types/masks.dart from types/types.dart
d2d763411d Use AbstractValue in inference results.
1c0b52ccc0 Reapply "Cleanup after removing MirrorsData" with fix
e491bc5d86 Allow to specify what dill file to initialize from
8101ee0479 [kernel] Remove unused GlobalIndexer from ast_to_binary.dart
dc8e621451 [js_runtime] Avoid more casts
0ad2b1243d Use carbon copy buffer field instead of passing it around.
6d55ba3037 [dart2js] Remove checks from inlined static call sites
6363cf6d3d Add new handleParenthesizedCondition event
393a47441f fix #33138, local class name should not conflict with inline-JS in SDK
6c3a861f8f [dart2js] Check arguments declared with generic method type parameter
499c25d886 [fuchsia] Update FIDL include paths.
d91af4625c Fix fix import URIs from file:// URIs on Windows.
53a51d17db Support for optional ImportPrefixGenerator to generate prefixes for new imports.
338a423416 Add Forest support for creating try statements
757ef3979c Revert "Revert "Revert "Don't do implicit new/const code generation with --preview-dart-2 enabled."""

* Update license.

* barback has been removed.

* Update license file.

* Update license file.

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Skia toString() is not maintained; the debugger that used it has been replaced. 
Replace usage with equivalent implementation.

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