Make test_sky work on Mac!

We still have a few tests failing, I believe
due to different font metrics.  Otherwise this works!

R=abarth@google.com

Refactor AnimationContainer to support drop-in Intentions.

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

Fix typo in README.md

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

Animate selected scrollable TabBar Tab into position

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.

Move sky package into sky/packages/sky

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

Previously test_sky was broken because it was still trying to use the
DEPS-based Dart packages.

Teach skyanalyzer about workbench

Hopefully this will make Travis green again.

Run sky:init before creating a snapshot

If we've never run sky:init before, the mojom package will be empty and the
snapshot will fail.

Update to mojo 402986f19a165b3be13108146a913245c2c36d7c

Remove DEPS-managed Dart packages

Improve init.dart not to assume `dart` is in your PATH

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

We now use `pub` to manage Dart packages.

Work around pub get going crazy

Fixes #306

TBR=johnmccutchan@google.com

Remove skygo

We no longer use sky_server.go. This patch removes it.

Break the dependency on Go-based sky_server

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

We now use the Dart sky_server in shelldb.

Teach gclient to run pub in more directories

At gclient sync time, we now run `pub get` for every Dart package except
`workbench`. This patch ensures that we have all the packages we need locally.

Avoid manually setting up in-process GL bindings for Skia

Pull a newer mojo_services

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.

Add bindings for "GL_NV_texture_barrier" required on Mac