Sundry debugging aids and fixes

Introduce a class that can describe the metrics used in a Positioned
widget, to enable easier manipulation and storage of the values, e.g.
for animation purposes.

(These are all the debugging-related fixes and trivial typo fixes that I
extracted out of my heroes branch.)

Fix rendering.dart import order.

Introduce a debugLabel for Performances so that when you create a
performance, you can tag it so that if later you print it out, you can
figure out which performance it is.

Allow the progress of a PerformanceView to be determined (but not set).

Allow subclasses of PerformanceView that are constants to be created by
defining a constant constructor for PerformanceView.

Introduce a debugPrint() method that throttles its output. This is a
test to see if it resolves the problems people have been having with
debugDumpRenderTree() et al having their output corrupted on Android. It
turns out (according to some things I read On The Internets) that
Android only has a 64KB kernel buffer for its logs and and if you output
to it too fast, it'll drop data on the floor. If this does in fact
reliably resolve this problem, we should probably move the fix over to
C++ land (where "print" is implemented) so that any use of print is
handled (avoiding the interleaving problem we have now if you use both
debugPrint() and print()).

Fix a bug with the debugging code for "size". In the specific case of a
RenderBox having a parent that doesn't set parentUsesSize, then later
the parent setting parentUsesSize but the child having its layout
short-circuited (e.g. because the constraints didn't change), we didn't
update the _DebugSize object to know that now it's ok that the size be
used by the parent, and we'd assert.

Also, allow a _DebugSize to be used to set the size of yourself.
Previously you could only set your size from a regular Size or from your
child's _DebugSize.

Add more debugging information to various Widgets where it might be
helpful.

Make GlobalKey's toString() include the runtimeType so that when
subclassing it the new class doesn't claim to be a GlobalKey instance.

Include the Widget's key in the Element's description since we don't
include it in the detailed description normally (it's in the name part).

Fix a test that was returning null from a route.

Fix scrollable tabs, add basic unit test

test harness for stocks app on iOS

Restored overlay_geometry example

Adds teleporting methods for moving sprite kinematic bodies

Animated sprite physics bodies now correctly transfers energy to dynamic bodies

The goal is to follow the guidelines in
https://github.com/flutter/engine/blob/master/sky/specs/style-guide.md#packages

Fixes #1638

Export new InputEvent classes from appropriate places

It's OK to just specify the colors list.

Fixes SpriteBox to work better with native points

Improves sprite physics debug drawing

Arena sweep

Fix warnings seen when running skyanalyzer in strong mode

Teach dumpRenderTree() to draw actual trees.
Make the TextStyle output terser so it doesn't overflow the output.
Make debugDumpApp() say what mode we're in (checked vs release).
Hide lifecycle state from release mode dumps (since it's checked-only state).

Fixes touch handling and refactors sprite physics

We had a typo in our CSS translation.

Implement better synchronization between the state of the input widget and the keyboard IME

Add a ListItem widget

For those times where you want to spin something around a point relative
to the size of your box, but you don't know the size of your box.

Also add an input widget API for the keyboard submit button