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

These are called selection controls in the spec and in the gallery UI.

This gives developers a breadcrumb they can follow to customize the
theme of their app. Previously, some customers were trying to set a
theme by manually configuring the colors of all their widgets.

Fixes #2980

Polish Slider implementation

Snackbar opacity animation shouldn't trigger on reverse

Based on feedback from the material design team.

Fixes #1613

Tab updates

Fixes #617

Update engine

Make it possible to dismiss a snackbar manually