More dartdoc for routes.dart

Assert binding ready before getting a11y listener

This adds an assert to verify that we have already got a binding when we
get a semantics listener. Otherwise we crash. (I mean, we still crash,
but now in checked mode we crash on an assert instead. Progress?)

Added support for Dismissable leave-behind list items

remove the _embedder.yaml file

implement --watch for flutter analyze

Cleanup around viewports

Cleanup build

Improve message when target not found

SizeObserver crusade: Remove SizeObserver

https://github.com/flutter/flutter/issues/2368

Include more details in the Red Box of Doom

Synchronizes dartdoc with dart:ui

Mostly just adds some examples to match the examples with use for
similar concepts in dart:ui.

Remove the second argument to WidgetError.

Remove the second argument to MultiChildLayoutDelegate.performLayout()

Fixes https://github.com/flutter/flutter/issues/2403

I have an e-mail ready to send to flutter-dev about this.

This makes WidgetError more like RenderingError, which will aid with https://github.com/flutter/flutter/issues/2356.

Fixes https://github.com/flutter/flutter/issues/2443

Factor out non-Material parts of MaterialApp into WidgetsApp

Remove Simulatable and restyle simulation_group.dart

Clarify the assert when mimicking twice in a row.

Fixes https://github.com/flutter/flutter/issues/1346

Try to clarify that you need a Material for an IconButton.

Flinging continuously should have one scroll start/end pair

Previously we got confused and started sending start/end pairs for each tick of
the fling animation.

Fixes #2430

Some more dartdoc

Fixes https://github.com/flutter/flutter/issues/2369

Clarify the "needs an Overlay" assert.