Make the Mozart demo more like a real window manager

Provide a better error for setState on a defunct State

We now have a resizer and close button. Also add some shadows and
launcher buttons.

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