Last dev push for `1.18`.

* send ext.flutter.debugDumpApp; ext.flutter.reassemble

* expose debugDumpRenderTree

We now stop updating semantics when there are no remaining clients.

Host apps will return null if there is no matching handler for a message name

Also, make SemanticsOwner into a real class and use it instead of a static in
several places.

Now that protected can be accessed from inside the same library, we can
use protected in a number of new places and we can remove some
trampolines we were using to work around its previous semantics.

This patch updates the framework to use the new SemanticAction in
semantics.mojom.

It can cause trouble if the user has a newer Android SDK installed normally.
Instead, just use the normal Android SDK.

navigating away.

Details are in the bug, but when we navigate away, the overscroll
indicator around the Scrollable would change the widget hierarchy. This
would trigger the Scrollable to rebuild, which would cause its
scrollOffset to be clamped to 0 for a frame.

BUG=https://github.com/flutter/flutter/issues/4597

Update to latest analyzer (w/ cleanup).

Bye-bye `@protected `trampolines! :)

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.