Update to latest Dart SDK (1.18.0-dev.3.0).

Our previous approach to detecting when we needed to remove semantics nodes
didn't account for reparenting.

Cleanup empty statements.

The new `empty_statements` lint (in the next DEV roll), flags these empty statements.  Harmless, except the one in `basic.dart` that fixes a real-life bug. :)

Previously these widget had zero test coverage. Also, improve the
coverage tooling Mac.

This patch adds tests for some code paths we weren't hitting before and removes
some dead code that couldn't be tested because it was unreachable.

These docs complete the dartdocs for their respective libraries.

* 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.