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

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

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

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

The workbench package exists to pull in pub packages from both pub.dartlang.org
and from the source tree (using dependency overrides). The idea is that
workbench will reflect a typical Sky developer's environment so that we can use
the same tools as a typical sky developer by running them with the workbench as
the cwd.

This package contains the mojom.dart files we generate from the sky_engine
repository.

After this patch, we use pub to manage versioning and fetching the material
design icons.

The Python HTTP server was caching too agressively. Instead, use an HTTP server
written in Dart from the sky_tools package.

Also fixed roll_versions to no longer roll mojo paths
since those are not part of the sky_engine repository.

R=abarth@google.com

NOTE: This CL appears far larger than it actually is for two reasons:

1) Many files were moved around to use the Dart package directory structure.
2) Many .dart files had to have import paths updated.

- Organize mojo/public/dart so that it uses standard Dart package layout
- Organize mojo/dart/apptest so that it uses a standard Dart package layout
- Organize sky/sdk so that it uses a standard Dart package layout
- Create a mojo/testing package (used by unittests)
- Introduce the 'dart_pkg' gn rule which populates gen/Config/dart-pkg
- All internally vended Dart packages must have a corresponding dart_pkg rule
- It is now possible to use dependency_overrides: in pubspec.yaml to mix internal and external package dependencies (enables analyzer, editor, webstorm usage for internal developers).
- Package root for dart content handler ends with "packages/"
- Imports of mojo package uris no longer need the "public/dart"
- mojo/public/tools/dart_package.py is a clone of mojo/public/tools/gn/zip.py
- Sky tests no longer run 'deploy_sdk' script.

R=eseidel@chromium.org

Review URL: https://codereview.chromium.org/1132063007

This CL does the following:
- Moves mojom's generate.dart from bin/ to lib/ to make it available for use
  by a wrapper scripts in the Sky package, thus avoiding all consumers of Sky
  needing to directly depend on mojom to be able to call
  "pub run mojom:generate".
- Adds a sky->mojom pub dependency and creates a wrapper script around mojom's
  generate.dart in the Sky package.
- Augments the Sky README to explain the usage of this script.

R=eseidel@chromium.org, sethladd@google.com

Review URL: https://codereview.chromium.org/1136503002

TBR=hansmuller@chromium.org

Review URL: https://codereview.chromium.org/1073243002

TBR=hansmuller@chromium.org

Review URL: https://codereview.chromium.org/1074933003

TBR=ojan@chromium.org

Review URL: https://codereview.chromium.org/1038873005

Also bumped package:sky's version in prep for publishing.

R=abarth@chromium.org
BUG=

Review URL: https://codereview.chromium.org/1024243002

1) Add a pubspec.yaml.
2) Move all the code into a 'lib' directory.
3) Move the stock widgets out of the app's library.

TBR=eseidel@chromium.org

Review URL: https://codereview.chromium.org/1011023003

The packages produced from this CL were deployed
as v0.0.1:
https://pub.dartlang.org/packages/sky
https://pub.dartlang.org/packages/mojo

There is still no tool included with the sky
package due to pub's inability to run anything
other than Dart:
https://code.google.com/p/dart/issues/detail?id=22877

I'm likely just going to write a dart version
of my "sky" script. :(

R=ojan@chromium.org, abarth@chromium.org

Review URL: https://codereview.chromium.org/1015833002