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.

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.

Scoped focus, so you can have a dialog with input controls and not lose your focus in the background.

This introduces a GlobalKey registry so that you can tell when a key
has gone away (so you can unfocus dead dialogs).

Also I added an assert that you're not calling setState() during a
build. It turns out that doing so means you have a bug, because since
you're dirty already (you're building), you won't get rebuilt.

The focus code itself is gnarly. It uses a Component and an internal
Inherited TagNode to manage the focus state, and does crazy things
like updating its state during build to pretend retroactively that it
was built with some other state, once someone asks for focus the first
time (i.e. the first time it's examined, so you can't tell that it was
in a different state before). It does this so that it can autofocus
controls which otherwise wouldn't be focused.

This moves all the focus management into Navigator and showDialog(),
so the complexity is all buried deep and not visible to apps,
hopefully.

To do something like two tabs that each have an Input widget that
needs to be focused when you switch panes, you'd need to have two
Focus objects, one in each tab, and you need to set their autofocus to
true (maybe that should be the default?).

@abarth doesn't like Futures that explicitly point out they're type-unsafe. :-P

Fix the crash for buttons.

Turns out there was a typo in lerpColor.

Fixes #277.

Use the navigator to stack dialogs.

This removes the need to manually include the dialog builder in the main window's build() function.
It also removes the need to track if a dialog is visible.

Other changes:
- I made dialog.dart a bit more readable.
- I renamed transitionFinished to fullyOpaque since that's what actually matters.
- I made Routes track if they're opaque. Eventually this should probably be more configurable when creating the route.

Directions for Future Research:
- Use this for focus management somehow.
- The popup menu should use something like this.
- We should factor the following out into a showDialog() function that returns a future for the dialog's exit result:
    navigator.push(new DialogRoute(builder: (navigator, route) { ... }));
- Maybe navigator.pop() should take a value to return to that Future.

Examples as libraries

Optimizations for Particle Systems

Create //sky/packages/workbench

Converted demo launcher example to library

Converted fitness example to library

Converted hello world example to library

Converted mine digger example to library

Converted stocks example to library

Conflicts:
	sky/sdk/example/game/lib/particle_system.dart

Use Skia's drawAtlas now that we've rolled to a newer Skia

Roll was ff8bbe4e1674ae57f2ea3a2841a37a9c099beac8
This is another step in fixing #138

R=viktork@google.com