Create wrapper script around mojom's generate.dart in the Sky package.

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

packages/flutter/bin/init.dart

+// Copyright 2015 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+/*
+* This script should be invoked via 'pub run' after 'pub get':
+* $ pub run sky:init
+* NOTE: The 'dart' executable must be on your $PATH for this script to work.
+*/
+
+import 'dart:io';
+
+main(List<String> arguments) {
+  ProcessResult result = Process.runSync('dart', ['-p', 'packages', 'packages/mojom/generate.dart']);
+  stdout.write(result.stdout);
+  stderr.write(result.stderr);
+}

packages/flutter/pubspec.yaml

@@ -5,3 +5,4 @@ homepage: https://github.com/domokit/mojo/tree/master/sky
 version: 0.0.5
 dependencies:
   mojo: '>=0.0.1 <1.0.0'
+  mojom: '>=0.0.4 <1.0.0'