Switch sky_tool to use a Dart-based HTTP server

This is supposed to make Viktor's game faster, but it's not clear
to me that it actually does.  I've left the code
using the dart version of drawAtlas for now until Viktor can
verify that it looks correct.

I also added a wrapper for SkFilterQuality in the process of
debugging SkCanvas.drawAtlas since all drawAtlas examples
in Skia use FilterQuality.low.  The bug which blocked me for
so long turned out to be that SkCanvas.drawAtlas doesn't
draw anything if antialiasing is turned on.

Issue #138.

R=abarth@google.com

RenderConstrainedBox was ignoring its additional constraints for its intrinsic size logic when it had no child.

Fix analyzer warnings

 - Remove unused imports
 - Remove unused, broken constructor for MineDiggerApp

This patch provides a class to manage focus, Focus, and a class to
manage ownership of the keyboard, KeyboardHandle.

Inherited (in widgets.dart) is adjusted to support subclasses that
need to copy state from instance to instance.

A GlobalKey key type is introduced that is basically the same as
UniqueKey. Component classes that need a globally-unique key can
specify that their 'key' constructor argument is a GlobalKey.

Focus
-----

You can use `Focus.at(this)` to determine if you, a Component, are
currently focused.

You can use `Focus.moveTo(this)` to take the focus or give it to a
particular component.

For this to work, there has to be a Focus class in the widget
hierarchy.

Currently, there can only be one Focus class, because nested scopes
aren't supported. We should add support for that in a future patch.
See issue #229.

KeyboardHandle
--------------

Instead of directly interacting with the keyboard service, you now ask
for a KeyboardHandle using `_keyboard.show(client)`. This returns a
KeyboardHandle class. On that class, you can call `handle.release()`
when you want to hide the keyboard. If `handle.attached` is still
true, and you need to reshow the keyboard after the user hid it, then
you can can `handle.showByRequest()`.

The point of this is that the `keyboard.show()` method maintains the
invariant that only one KeyboardHandle is live at a time.

There are some issues with the keyboard service that I filed as a
result of doing this patch: #226 #227

Accessing the network created a race condition that would sometimes cause other
tests to fail.

The Python HTTP server was caching too agressively. Instead, use an HTTP server
written in Dart from the sky_tools package.

Fix the crash when going to settings after showing the popup menu.

Conflicts:
	sky/sdk/example/fitness/lib/feed.dart

If your constraints are tight when you get laid out, you don't get a
relayout subtree root.

If you don't have a relayout subtree root, and you get marked dirty,
you go through layoutWithoutResize() rather than layout(), so we don't
get a parentUsesSize.

If you're not dirty and your constraints didn't change, layout() skips
your layout.

So then if your initial layout had parentUsesSize:true, and then you
got marked dirty directly, you would set your size with
parentCanUseSize=false, and then later if your parent tried to lay you
out then read your size, it would crash because your size wasn't set
up to allow you to get your size.

The fix is to actually remember the last setting of parentUsesSize,
even in the case of the constraints being tight and you later being
marked as needing layout directly.

In theory, before, if you had the same image twice in a scrolling container, you'd get an assertion with no way around it.
This makes those nodes not bother making keys by default, which is cheaper and more correct.

Make SnackBar animate itself.

This introduces a bit of a regression - the FloatingActionButton no longer animates. I'm not sure yet how to animated the FAB along with the SnackBar. Maybe some notion of anchoring a node to another.

Also remove last use of AnimationBuilder.

This fixes some theoretical bugs whereby we were using hashCode to try
to get unique keys for objects, but really we wanted object identity.
It also lays the groundwork for a new GlobalKey concept.

I tried to keep the impact on the code minimal, which is why the "Key"
constructor is actually a factory that returns a StringKey. The code
has this class hierarchy:

```
   KeyBase
    |
   Key--------------+---------------+
    |               |               |
   StringKey    ObjectKey       UniqueKey
```

...where the constructors are Key and Key.stringify (StringKey),
Key.fromObjectIdentity (ObjectKey), and Key.unique (UniqueKey).

We could instead of factory methods use regular constructors with the
following hierarchy:

```
   KeyBase
    |
   LocalKey---------+---------------+
    |               |               |
   Key      ObjectIdentityKey   UniqueKey
```

...with constructors Key, Key.stringify, ObjectIdentityKey, and
UniqueKey, but I felt that that was maybe a more confusing hierarchy.
I don't have a strong opinion on this.

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.