* Change how navigator prevents redundant operations

Instead of requiring transactions, we now cancel all active pointers that are
interacting with the navigator and absorb future pointers until we get a chance
to build. This approach isn't perfect (e.g., events that trigger off the
cancelled pointers could still interact with the navigator), but it should be
better than the current transaction-based approach.

Fixes #4716

* Remove openTransaction

* test

* fixup

This patch documents State in detail because it's a very common class
for developers to work with and it has a number of subtles.

* Fix the issue that flutter driver does not close _peer when
driver.close() is invoked.  The problem introduces the following
unexpected behavior:

  1. Launch an app using "flutter run ..." command
  2. Run the flutter driver test using "dart flutter_test.dart"

The test will not exit after running.  The problem will be solved
if _peer is closed.

* Fix formatting issue

Previously this widget was completely untested.

The one line we don't test is the private constructor for the Curves class,
which exists to ensure that no one constructs the Curves class.

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).