Sporadic crash from invalid touch event

This required some changes to AnimationPerformance to better understand
animating with forces.

When embedded by the view_manager, sometimes we receive pointerup or
pointercancel events without having received a cooresponding pointerdown event.
The underlying issue is that the view_manager doesn't capture on pointerdown
and instead performs a new hit test for every pointer event. We should fix that
in view_manager, but, in the meantime, this patch makes us not crash in this
scenario.

Fixes #339

I incorrectly thought we were now supposed to import these mojoms from the
packages that publish them. However, we're still supposed to import them from
the virtual mojom package.

Also, update imports to account for new mojom.dart locations.

TBR=abarth@google.com

Adds star field to game start screen

Fixes analyzer warnings in sprites

Fix analyzer warnings

SnackBar, and PopupMenu instead of custom statuses.

Move build() off microtasks

Rather than using a microtask to schedule component build functions, instead
use the scheduler. We now tread building just like layout and painting as a
visual update.

Optimizations for sprite code

This lets us be flexible in how to animate the properties of the container.
Currently used by SnackBar with a SlideIn intention.

When the selected tab changes in a TabBar for which isScrollable:true, animate the selected tab to the center of the viewport, if that's possible.

Added an optional duration: to Scrollable.scrollTo(newOffset, duration: d) which animates the scroll to newOffset.

Refactored the Scrollable implementation a little to make the roles of the "toEnd" and "toOffset" animations clearer.

_handlePointerDown() now stops both animations.

Similarly, we only call settleScrollOffset() from the cancel gesture handlers if the animations aren't already running.

This patch makes the sky package match the structure we've adopted for the
other Dart packages we produce from this repo.

Remove DEPS-managed Dart packages

Instead, use Platform.executable, which is the path to the current `dart`
executable.

We now use `pub` to manage Dart packages.

Optimizations for Particle Systems

Refactors randomizations of color sequences and optimizes particle systems (uses single Paint instance, doesn't compute accelerations unless used, reduces number of created objects, faster color calculations)
Fixes indentation
Optimizes lookup in fast version of atan2 (toInt is faster than floor)
Removes frame rate printout and adds assert