Initial version of complex layout performance test

Tweak the typography demo to match the spec more

Previously we weren't padding the top and the bottom of drop-down menus but the

spec says these should have 8.0 pixel of padding, the same as popup menus.



Fixes #2683

Support "flutter refresh" in the Gradle-based sample project

* sort imports

Polish selection controls

Leave all entered targets when finishing.

pubspec normalization; remove den

Capture closures around megamorphic dispatches

The performLayout and build callsite are highly megamorphic because they
dispatch into a large number of clients. However, for a given caller, the
callee is always of the same type, which means the megamorphic lookup exactly
factors by the caller. We can speed up the dispatch by capturing a closure at
initialization and then monomorphically dispatching through the closure.

Add a scrolling test now that it passes

Update engine version

Don't delay between dismissing a snack bar and the next snack bar

Update engine

Previously, the Dismissable widget was animating down to zero width off
screen. Now, we tell it to skip the resize animation.

Fixes #3030

Add review feedback to dartdocs

Fix broken link in the docs

Drop is a separate word than down

We fixed this issue a different way but the test is still valuable.

[driver] switch to different pause event workaround

Fix spinning_mixed widget example, plus minor style fix

Improve setState-after-dispose error message

We now explicitly mention the possibility of a memory leak.

Fixes #2978

Therefore, we use _ between them in the file name.

Initial version of Fancy line drawer demo

Part 2 of independent layout pipelines

Adds BuildOwner to manage the dirty list and build processing for
widgets/elements, and adds a widget unit test to make sure separation
is enforced.

Fixes #2723

Rename ToggleControlsDemo to SelectionControlsDemo

Document which widgets require a Material ancestor

Also, fill out other documentation for these widgets.

Fixes #967

Be explicit about the theme in the starter app