Canceling a dropdown menu selects null value

Previously we didn't distinguish between canceling the menu (which
popped the route with no return value, i.e. null) and explicitly
selecting an item whose value is null (pop with value null). Both fired
onChange(null).

Now we box the return value so we can distinguish the two cases.

I would have preferred to just disallow "null" as a value but it turns
out people like being able to use "null" as a value for much the same
reason we do, and it seems best for us to pay the complexity cost of
boxing than having everyone else do it. :-)

Added BottomSheet demos to the Material Gallery

Tabs should only scroll on a horizontal TabBarView fling

Workaround to re-enable test

This is a workaround for:
   https://github.com/dart-lang/sdk/issues/25246

Use a newer version of dartdoc

Custom layouts use shouldRelayout() delegate methods instead of tokens

CustomMultiChildLayout and CustomOneChildLayout now use their delegate's shouldRelayout() method instead of a "token" to decide if layout is needed.

MultiChildLayoutDelegate and OnChildLayoutDelegate are now expected to be stateless, i.e. they'll typically be built each time their custom layout widget is built. If the identical layout delegate is provided to a new custom layout, layout will not happen.

Revised the bottom sheet implementation per the new custom layout classes. Removed a SizeObserver.

Fixes #899

CustomMultiChildLayout and CustomOneChildLayout now use their delegate's shouldRelayout() method instead of a "token" to decide if layout is needed.

MultiChildLayoutDelegate and OnChildLayoutDelegate are now expected to be stateless, i.e. they'll typically be built each time their custom layout widget is built. If the identical layout delegate is provided to a new custom layout, layout will not happen.

Revised the bottom sheet implementation per the new custom layout classes. Removed a SizeObserver.

Fixes #899

remove todos for service extension params

Export dependent class from newton package

expose flutter debug functionality using VM service extensions

Update test.sh

Add the -q (quiet) flag to gsutil uploads.

Update test.sh

gen dartdoc using the flutter styles

Switch docs to docs.flutter.io

That site is now live.

See if fixing indent fixes our missing caching

@abarth

rev engine

Fix formatting issue

Add nicer logging/reporting to dev/update_packages.dart

Makes it more obvious where we are spending our time on Travis.

Also added travis caching for the global pub cache to
speed this up!  Modeled what dartdoc does:
https://github.com/dart-lang/dartdoc/blob/master/.travis.yml#L13

@Hixie

Rename stocks/tests to stocks/test

This matches the naming pattern expected from package:test
`flutter test` doesn't care, since it finds all _test.dart
files and runs them regardless of directory.

@Hixie

Fix build breakage

Split out Instrumentation logic from WidgetTester

This will allow writing tests/benchmark which want to use
the engine's default beginFrame and normal passage of time.

@Hixie

Always import our packages when analyzing.