Fixes issues with invalidating matrix for SpriteBox & adds HUD to demo game

Many code improvements to Mine Digger.

Fix type error found by dartanalyzer --strong

Grammar

We were declaring that the function passed to setState should return a
Function. In reality, we want the function to return void (and everyone calls
it with a function that returns void).

Fix 2 crashes found by Hixie's fuzzer.

Update sky_tool now that sky:// isn't a thing

Instead, just use a normal http URL.

Text styles are now a global constant instead of being initialised dynamically.
There are now sufficient text styles for 8 mines around a square.
I coallesced Game and MineDiggerApp.
Used an enum instead of constants for the cell state.
Used setState() instead of scheduleBuild().
Used an O(N) algorithm for mine laying instead of the previous potentially-never-ending algorithm.
Moved the listener on the cells to the app, so that the logic survives a rebuild.
Various other minor changes.

The first one is that we weren't setting up a
FontCachePurgePreventer during drawText.  It's not clear
that this is the correct fix, since Blink doesn't have
this FontCachePurgePreventer here either, but it's also
possible that they would hit this same ASSERT and just
not care (since ASSERTs are disabled on clusterfuzz).

The second fix is making ExceptionState actually track
whether it has thrown an exception or not. The c++ code
was depending on this working in order to return early
from dom functions and not crash!

R=abarth@google.com

It turns out that we aren't really using these. The identity logic is sufficient.

Also, add some asserts for a crash I had once but couldn't reproduce, in case that helps catch it next time.

Route (named routes) no longer have a key, and have their own storage for their names.
RouseState no longer has a key, and uses an owner field pointing to a StatefulComponent instead.
As such, RouteBase no longer has a key.

HistoryEntry no longer uses a global int to ensure uniqueness.

Propagated this to stocks app.

When we build widgets during layout we weren't notifying those widgets that
they were mounted. Now we notify mount status changes when exiting build phases
inside layout.

Fixes #176

Separate width and height parameters for Image widgets

Fix analyzer warnings

This change makes it easier to defined only the width or the height of an image
and let the other value be filled in from the image's intrinsic aspect ratio.

Fixes #175

Renamed AnimatedType to AnimatedValue

Updates sprite widget to handle changes to renderbox

Dismissable: animate "snap back", only call dismiss() once