Fixes #1695

Actually populate the view host proxy

Add a "SLOW MODE" banner in checked mode

Remove ParentData#merge

There aren't any clients anymore.

Fixes #1684

Use the same flags as Android on iOS while starting on the simulator

Now that the engine lets us take the view host handle, we can actually
access the view host from Dart.

start the simulator with -arch x86_64

iOS Sim: Wire in launch arguments to simctl

Remove some redundant words.

Make flutter create generate a manifest that is similar to SkyShell.apk, fixes #1587

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.