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

This patch picks up a new url_loader.mojom interface that is needed to access
the network in production.

Cleanups to network service

This patch contains some minor improvements to the network service:

- We now cache the connection to the network service so we don't need to spin
  it up for each request.
- We now manage the lifetime of NetworkServiceImpl in the same way as other
  services.

Also, update the workbench sky_tools dependency to the latest version.

Teach big_red_button how to publish sky, sky_engine, sky_services

This CL makess big_red_button.py work for the sky, sky_engine, sky_services
packages.