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.

fix flutter create

Fixes https://github.com/flutter/flutter/issues/2436

[infra] run recipes using recommended method

Factor MojoShell into shell.dart

Make _initRootBundle more robust

Teaches ChildViewConnection to wrap an existing ViewOwner

fix a mac test

Putting this code in a separate library let's services that depend only on the
shell be clearer about their dependencies.

Fixes #1921