Fixes https://github.com/flutter/flutter/issues/14555

Inlcudes:
* Enable -Wunguaraded-availability for iOS builds (flutter/buildroot#126)
* Eliminate is_nacl from build config (flutter/buildroot#127)

/bin/sh, for example, doesn't support pushd/popd.

* Track widget creation locations.

Place all iOS code that relies on APIs introduced in iOS versions later
than our base SDK version (iOS 8) behind @Available checks. This allows
us to enable the -Wunguarded-availability compiler flag for iOS builds
in the buildroot repo.

* Switch to accept/reject-friendly incremental compiler

* Merge

* Use fallback root certificats on Windows, Linux host builds

* Use fallback only on Windows

* Roll Dart to 223eeb2ebe112aaaddca206aab55cd54b4e54391
* Update licenses
* Make analyzer happy, fix test for change in mockito

It seems that setTraversalBefore doesn't work as well as setTraversalAfter for some reason, although I'm using them the same way. Some apps would lock up TalkBack when traversing if setTraversalBefore was set, but not with the equivalent setTraversalAfter.

It's not entirely clear why this is, but I'm going with this to at least get it fixed for apps we know about.

Addresses flutter/flutter#14600

See also flutter/flutter#14607

Includes https://fuchsia-review.googlesource.com/c/topaz/+/121856, which
enables path sanitization for Dart source loaded directly from from
file:// URIs (e.g., main entry points from flutter_tester).

This re-enables whitelisting of specific filetypes to avoid diffs
including irrelevant changes in case of formatting issues. It also fixes
a missing exit code.

* Make native wrapper classes non-abstract as they are instantiated from native code
* Make constructors of native wrapper classes private

Lands
https://fuchsia.googlesource.com/topaz/+/046105efd225ed61c94e65dbd4a528256a3a34a9
which adds support for general handling of URI %xx escape sequences.
This enables support for paths including accents, CJK characters, emoji,
etc.

This picks up fuchsia/topaz@fe3f1503dcc2b40ce0b7a3a1dffba7a58126607e,
which ensures paths resolved from package: URIs are properly sanitized for
correct handling of URI-encoded path components.

* Roll dart deps

* Remove pin

* Roll buildroot to dart dev.20

* Let frontend_server bootstrap the incremental compiler from a previously build dill file

* Wrap 'invalidateIfBootstrapping' in try and don't bootstrap if getting an exception

This relands https://github.com/flutter/engine/pull/4629 with a
tonic roll to fix the build.

* Roll flutter to 06-02-2018 402e69b53d59083d34d7538126837c4381aa0c4a

* Change license signature

* implement iOS text field editing in a11y mode

* address Chinmay's comments

* replace node in child list when changing type

* Fixes to make https://dart-review.googlesource.com/c/sdk/+/36641 work with Flutter

* Roll dart

* Update license

* Roll dart

* Update license

* Add libraries.json to dependencies

This reverts commit 8f638591.