This ensures we don't run into discontinuities when reversing an
animation halfway through. I refactored AnimationValue to have knowledge
of the reverse curves and intervals.

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

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