Refactor PerformanceView listeners.

Primarily, this rearranges how listeners are handled internally by the
various PerformanceView classes so that there's not so much redundancy.

It also fixes ReversePerformance and ProxyPerformance to not leak.
Previously, they never unhooked their listeners, so they'd leak until
the entire chain of performances was killed. Now, they unhook as soon as
they themselves have no listeners left, which is more idiomatic.

Improve the asset for build functions that return null

Add more asserts to help catch the blackness bug

Fixes #689

Support for hero transitions underneath the target page

Refactor forward transition building

Instead of separate functions for the 0->1 and 1->2 transitions, just
have one function.

MultiChildLayoutDelegate should assert that all children have had layoutChild called exactly once

Make profile_startup.dart print instructions.

BuilderTransition no longer requires variables

Fixes #692
Fixes #690

@chinmaygarde

Sometimes you want to just make a transition driven straight from the
performance's progress, rather than with an explicit variable.

Add custom clipping to ClipRect and ClipOval

Add the ability to custom paint foregrounds

Add script for profiling startup using flutter trace on Android

Slider Improvements

Have the widget support min/max arguments to make the widget easier to
use. Also, fix the dragging so it's actually where the knob is.

This will work after the next Engine roll.  Currently it requires
a fix to tracing which isn't in the released engine.

Next steps are to make this work on a bot, and to add iOS support.

@abarth @chinmaygarde

Example of using the Dart internationalization package in Flutter

Improve material ink response

This patch contains a number of improvements to the material ink response:

- The ink response now remains until you lift your finger
- When disappearing, the ink response now fades out
- The ink response is now the correct color (at least in the light theme)
- The ink response for IconButton now has a (circular) highlight
- The ink response for IconButton now repositions itself to be centered on the highlight

In addition, I've adjusted the various animation parameters to better match the
behavior of ink responses in the Java implementation of material.

Fixes #695

Introduce some basic utility classes.

Analyzer improvements

One might argue, the most basic utility classes.

- strengthen analyzer against error messages that point past the
  bottom of the file.

- ignore any errors in intl's auto-generated files.

Automatically fill in the menu button when there's a drawer

Add support for letterSpacing to TextStyle

Also, fill in a back arrow when we can go back.

Fixes #699

Workaround logcat -c not working