Preliminaries for scroll anchoring

These classes now share more code and have feature parity.

Gallery Demos: button, dialog, snack bar

Added button, dialog, and snack bar gallery demos.

Split gallery main.dart into files in the lib/gallery subdirectory.

Enable TabBarSelectionState to deal with the PageStorage saved selected value no longer equalling any value.

Changed the color of Dialog text to match the theme's caption color.

Also: uppercased tab labels per #1617

Fix drawer in accessibility mode

The "leaf merge" feature was getting confused when we reset the
SemanticsNode. We now separately track whether the node itself is marked
as being merged vs whether we inherited that state, and we don't reset
the inherited state until you're reattached or reserialised. In the
latter case, we do a "just in time" clearing of the flag just like we
previously did a "just in time" setting of the flag, except now the flag
we're setting or clearing is the inherited flag not the actual flag.

Fix `flutter stop` to stop the right Android activity.

Previously we'd always stop org.domokit.sky.shell. If an
AndroidManifest.xml exists, we'll use the activity specified in there
instead.

Skip flutter_simple in create_test on Mac for now

Returns 69 instead of 0 in the `create --out` part.
https://github.com/flutter/flutter/issues/1709

@devoncarew

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