* Handle error count reported by frontend.

Extend compilation result from single string to a structure(string filename and integer error count).

* Use ?.

* Include engine roll with dart sdk roll.

* parse(onError) -> tryParse

* Make '?? throw' more readable and avoid issue with analyzer

* Fix test so it mocks compiler output including errors count

This fixes a rendering problem in the AnimatedChildSwitcher where it would add a new "previous" child each time it rebuilt, and if you did it fast enough, all of them would disappear from the page.

It also expands the API for AnimatedChildSwitcher to allow you to specify your own transition and/or layout builder for the transition.

Fixes #16226

This reverts commit 6494ab23f6a063c689441f55c3aba1b3c8e26b5b.

* fix a test flake on appveyor

* only catch FileSystemExceptions

* A new UI is being designed to make the unused reload messaging clearer,
the UI will use the coverage data to highlight lines in the UI that were not executed during a reload.

In lieu of that we are turning off the unused reload messages experiment.

There were some issues in Dart2 also with this functionality
- source fingerprinting is not implemented in Dart2
- some additional synthetic functions are generated for invocation argument checking that are not appropriately filtered out.

This converts the chips to use a ChipThemeData to get most of their customization values from (if not overridden by specific arguments to the chip constructors), and to have the base ThemeData contain one of these. It also adds the ChipTheme widget that will allow overriding the theme for a particular subtree in the widget hierarchy.

Added tests for both, and just more tests in general for the Chips.

The spec changed the size of the slider thumb, so this just effects that change.

When flutter builds are run with --verbose, pass the flag through to
flutter sub-commands run via xcode_backend.sh.

* Introduce fingerprint check for preview-dart-2 build flx.

* Don't count outputs into fingerpint. Refactor compilation code.

Inlines the very small amount of work being done in _build() into
buildScriptSnapshot(). Eliminates a duplicate (and un-awaited) call to
_writeFingerprint.

Revert "re-write flutter analyze (the single-shot and --flutter-repo) to use the analysis server (#16281)" (#16482)

This reverts commit 2f41ea54.

re-write flutter analyze (the single-shot and --flutter-repo) to use the analysis server

* Sync .packages file as regular file.

Currently .packages file is treated specially as DevFSStringContent to accommodate package-file rewrite when it is sent to the device for dart1 compilation. In dart2 we need to treat .packages as regular file because from frontend perspective it's just a normal input file.

Revert 9749bea7. Accidentally pushed to flutter instead of fork repo for PR.

This reverts commit 9749bea7.

Fixes https://github.com/flutter/flutter/issues/16346.

See https://github.com/flutter/flutter/issues/16346#issuecomment-380255095 for detailed explanation of what was going wrong before this fix.

Changes since last roll:
```
ed303c628 Roll dart sdk again. Previous roll required 23ae4fa098 revert. (#4966)
8cd272733 Revert "Roll dart to 7764e6962e22afcf4b58c4e3cef3147330f3c884. (#4960)" (#4965)
9199b40f2 Revert "Support multiple shells in a single process. (#4932)" (#4964)
6baff4c82 Support multiple shells in a single process. (#4932)
31c5bb427 Roll dart to 7764e6962e22afcf4b58c4e3cef3147330f3c884. (#4960)
c8e4c6984 Avoid copying the contents of large platform message responses (#4947)
5ff527295 Update to use new vulkan GrBackendRenderTarget ctor. (#4962)
0c8993a1a Update to use new vulkan GrBackendRenderTarget ctor (part 2) (#4963)
132ebdda8 Revert "Roll src/third_party/skia/ 9874bf1bc..52e16d984 (135 commits) (#4958)" (#4961)
11882ab9e Roll src/third_party/skia/ 9874bf1bc..52e16d984 (135 commits) (#4958)
```

Add consts

* Update bin/internal/README.md

* Update README.md

See https://github.com/flutter/flutter/issues/12630 and
https://github.com/flutter/flutter/issues/16408

There was a bug in the size calculation for the height of the chip label, where it would force the label to be larger than it wanted to be, causing text to not be vertically centered.

* do not add run spacing if there are no run metrics

Make the full-screen dialog text editable in gallery (#13690)

This fixes https://github.com/dart-lang/sdk/issues/32598.

We are about to begin building gen_snapshot as a multi-arch binary,
which when run as x86_64 will generate arm64 AOT output, and when run as
i386 will generate armv7 AOT output.

Currently, gen_snapshot is an x86_64 binary, so this change is
effectively preventative in nature, and is a no-op with the current
snapshotter.

* Update text_style_test