General flutter_tools refactoring

remove the device factory methods as well as the device cache

two fixes for checked mode

add a dev mode to the flutter script

Overlay polish

Remove ColorTransition

Fixes #1607

Remove HomogeneousViewport

There are no clients anymore. We added it for the Drawer a while ago,
which is a use case that's now better covered by AnimatedModalBarrier,
complete with semantics.

Add more dartdoc to widgets.dart

This patch includes documentation for transitions.dart and for
scrollable.dart.

The virtual viewport machinery now handles all of these use cases.
Previous clients of ScrollableWidgetList can use ScrollableLazyList
instead.

Watch sim devices

add device notifications for the simulator

Alphabetize component demos

Create flutter_tools.dart to represent public tools API

Gallery support for light/dark theme switching, slow animation speed

Improve exceptions and asserts for rendering lib.

* Use actual exceptions rather than assertions containing code
  containing strings when trying to give messages to authors.
* Introduce RenderingError which is an AssertionError that takes a
  string argument, to support the above.
* Provide a BoxDimensions.hasBoundedWidth/hasBoundedHeight API.
* Document BoxDimensions.isNormalized.
* Provide more useful information when we assert isNormalized and find
  that it is false.
* When finding the size is infinite, crawl the tree to figure out which
  render box is likely responsible for the infinite constraints.
* Provide more information when size doesn't match the constraints.
* Provide more information when intrinsic dimension methods violate the
  constraints.
* Only spam a huge amount of information for the first exception from
  the rendering library. I've noticed a lot of people looking at the
  last exception printed rather than the first and that's very
  misleading -- after the rendering library hits an exception, all bets
  are off regarding what'll happen in the future. All kinds of asserts
  might fire.
* Improve docs around the debug methods and flags for the above.
* Make Block default to have no children. Previously, giving no children
  crashed with a confusing message about a null deref in an assert.

Add more dartdoc to widgets.dart

Making progress documenting the widget library.

Support TabLabel widgets

Add some more dartdoc to gestures.dart

Add more dartdoc to animation.dart

After the refactoring of the animation library, we were missing some
dartdoc.

Bump the dart version used on the infra bots to 1.14.1

Refactor flx.build() to allow for external toolchain hooks.

Otherwise the analyzer blows up.

@abarth

This splits flx.build() into two methods, flx.build() and
flx.assemble().  builD() now does the following:
1) constructs the manifest map by reading the manifest from the
   file system
2) "compiles" the dart code into the snapshot file
3) Invokes assemble()

This allows external build toolchains to construct their own
manifest map (possibly using a different manifest syntax)
and create their own snapshot file

Add a download_dart_sdk.py script for infra bots

remove no longer needed code from the service protocol extensions