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

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.

Complete OpenGL setup and input event handling on Mac

Enable scrollable tabs fling scrolling

We need to pull in an updated version of sky_tools to get a version that
doesn't try to gzip everything.

Fix sky_tool's installing of SkyShell.apk

We were walking over to the realdir too quickly, which meant we lost which
version of the sky_engine package we were supposed to use.

Teach ./sky/tools/gn where to run the gn binary

Remove Location.idl and assoicated code

Remove Screen.idl and associated code