We're going to remove DrawLooper from the canvas API soon.

Fixes #2198

The mojo_sdk has cleaned up its dependencies, so we no longer need to depend on
package:test from package:flutter.

This patch switches us from using our previous hacking approach to integrating
with package:test to using the new PlatformPlugin interface.

Fixes #2361

This test causes the buildbot to hang. We don't currently have any hang
protection, which means this test ends up killing the bot. Disable the
test for now why we work on the root issues.

This change brings in a new version of Mojo, which has improved Mozart
interfaces.

Fixes #2337

Fixes #2324

This enum doesn't make sense anymore now that we can arbitrarily colorize
icons.  Instead, we just use a Color, which is both simpler and can be
interpolated during animations.

Fixes #1279

Makes it easier to use without a SizeObserver

Rather than managing all the Material Design icons manually, we now
manage them using an icon font. The icon font contains glyphs for each
icon in an efficient vector format.

This patch updates the FLX tooling to include the MaterialIcons font and
updates the Icon widget to use the font instead of asset images.

Fixes #2313
Fixes #2218
Fixes #2009
Fixes #994

Fixes #1381

Now we produce an error message that explains why this operation isn't
permitted by the framework.

Fixes #1925

* Makes repeat default to the lower and upper bounds
* Makes animateTo handle unbounded ranges better
* Improve the name of _TweenSimulation

Fixes #2096
Fixes #2090
Fixes #2091

This concept is now private to AnimationController. All the clients actually
want the AnimationStatus.

Previously we would elide forward and reverse callbacks that canceled each
other out, which broke the expected state machine. Now we synchronously deliver
status callbacks when start an animation.

Fixes #1913

This assert was overzealous. You can update a RawGestureDetector any
time before the semantics phase, it doesn't have to only be during your
child's layout.

This is good because Block has additional PointerListeners between its
GestureDetector and its Viewport.

Now onScroll callbacks are always bracketed by onScrollStart and onScrollEnd
callbacks.

Fixes #1822

Somehow this actually made heroes way simpler.

We don't need these keys. Also, improve style in several places.

Fixes #2225

...and friends.