Fixes #608

Implement ImageRepeat

Cleanup for the Pointer changes.

Fixes #158

PointerInput refactor

Instead of PointerInputEvent having a "type" field, we now have a
different class for each pointer type.

This has ripple effects throughout the system.

I also did code cleanup in affected files while I was there.

Remove PaintingCanvas

Remove outdated radial_reaction.dart

We can just use Canvas now.  The two are the same thing.

We now draw radial reactions using Material.

Replace AnimationTiming with CurvedPerformance

MeanPerformance

Also, replace places that use the term "create performance" with
"create performance controller".

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