* General improvoments to the loader app:
   * Show a message after 8 seconds if no connection comes in.
   * Show a progress bar as files are being uploaded.
   * Hide the spinner just before launching the application.

* General improvements to the "flutter run" UI:
   * Add "?" key as a silent alias for "h".
   * Make the help text bold so it doesn't get mixed with the logs.
   * Make "R" do a cold restart when hot reload is enabled.

* Supporting features and bug fixes:
   * Add support for string service extensions.

* Other bug fixes:
   * Expose debugDumpRenderTree() outside debug mode.
   * Logger.supportsColor was missing a getter.
   * Mention in the usage docs that --hot requires --resident.
   * Trivial style fixes.

This script runs the Flutter unit tests. By default, the script assumes you
have compiled a SkyShell in an "engine/src" that's a peer to the "flutter"
directory.

This command will eventually become the main entrypoint for the Flutter tools.
Also, fixup a reference to the "sky" package in sky_tool.

We haven't used init.dart in several releases. We can safely remove this no-op
script now.

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

I'm not sure this is the final long-term solution, but works for now.

R=abarth@chromium.org

Review URL: https://codereview.chromium.org/1022193002

TBR=abarth@chromium.org
BUG=

Review URL: https://codereview.chromium.org/1021933004