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

Rather than catching every exception, we now handle the one specific case where
we legitimately cannot create the root bundle.

Fixes #900

Add dartdoc to VirtualViewport widgets

Remove dependency on material_design_icons

Random fixes to dartdocs, http lib, analyzer

We get the icons from the iconfont now.

Also, remove the package:flutter CHANGELOG.md.  This file is outdated and the
git history is a more accurate change log these days.

This is a variety of trivial fixes that I collected while going through the dartdocs.
This does contain some non-comment changes.

Update engine.version

Add tests for AssetVendor and resolution-dependent image loading

Remove the one use of DrawLooper

We're going to remove DrawLooper from the canvas API soon.

[ios] use xcrun to launch simulator instead of hardcoded path

Container's child is now inset by the Container's border