This patch improves some subtle behaviors about the change notifier.

* send the reload source command for ios simulators

* review changes

* rewrite w/ a completer

`flutter test` now has a `--merge-coverage` flag that can be used to merge
coverage data from previous runs, enabling faster iteration cycles.

We now download the base coverage file automatically.

Stop spamming the console about drudge by pinning an earlier version of
json_scheme that doesn't spam.

Using relative paths makes it easier to share lcov.info files across machines.

We now have 100% coverage of animation.dart and animation_controller.dart.
Also, add some basic tools for working with lcov files. These tools need much
more polish.

Uploading the coverage data file to Google Storage will make it easier
for folks to get current coverage data quickly.

Update to latest analyzer (0.27.4-alpha.14) and misc. analysis fixes.

It's not reliable enough yet.

Also, fix the bug where apps would assert when you change the time dilation.
Use that mechanism to fix the animation controller tests.

#4672

This patch causes us to upload a coverage report to coveralls.io.

* brings in analyzer version (`0.27.4-alpha.14`) corresponding to current Dart SDK (`1.18.0-dev.2.0`).
* updates analysis to use prefered API for embedder URI resolution
* adds trampolines to `State` and `StatelessWidget` to allow for warning-free within-library @protected access (needed since we closed off access to @protected closures from outside subclasses).
* turns off cache dependency tracking for analysis (in DDC this amounted to a 10% speed improvement).

We need https://github.com/dart-lang/coverage/issues/100 to be fixed before
this will be useful.

Fixes #2342

Bump Dart SDK to `1.18.0-dev.2.0`.

This API is the front-end part of the work on showing licenses.

Future patches will:

* Provide an API for registering what licenses should be shown here,
  which will be used by this feature to shown licenses but could also be
  used by custom code for showing licenses (e.g. for people not using
  the Material widgets).

* Actually populate this license API from all the licenses we currently
  use in the engine, in the framework, and from any pub packages that
  are used (directly or indirectly) by the application.

Gets a bunch of fixes.  Most notably a VM that has background compilation re-enabled (and stable).  This should win us (back) a ~17% improvement of analysis times (CLI and server).

See: https://github.com/dart-lang/sdk/issues/26697 for more context.

I erroneously removed this function in an earlier patch. Also, export
SystemUiOverlayStyle because we expose that enum in the SystemChrome
API.