Add a `flutter upgrade` command

This command upgrades the version of flutter that you're using and runs
`pub upgrade` to update the transitive dependencies.

packages/flutter_tools/lib/executable.dart

@@ -23,6 +23,7 @@ import 'src/commands/start.dart';
 import 'src/commands/stop.dart';
 import 'src/commands/test.dart';
 import 'src/commands/trace.dart';
+import 'src/commands/upgrade.dart';
 import 'src/process.dart';
 
 /// Main entry point for commands.
@@ -57,7 +58,8 @@ Future main(List<String> args) async {
     ..addCommand(new StartCommand())
     ..addCommand(new StopCommand())
     ..addCommand(new TestCommand())
-    ..addCommand(new TraceCommand());
+    ..addCommand(new TraceCommand())
+    ..addCommand(new UpgradeCommand());
 
   return Chain.capture(() async {
     dynamic result = await runner.run(args);

packages/flutter_tools/lib/src/commands/upgrade.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.
+
+import 'dart:async';
+
+import 'package:logging/logging.dart';
+
+import '../artifacts.dart';
+import '../process.dart';
+import 'flutter_command.dart';
+
+final Logger _logging = new Logger('flutter_tools.upgrade');
+
+class UpgradeCommand extends FlutterCommand {
+  final String name = 'upgrade';
+  final String description = 'Upgrade your copy of Flutter.';
+
+  @override
+  Future<int> runInProject() async {
+    int code = await runCommandAndStreamOutput([
+      'git', 'pull', '--ff-only'
+    ], workingDirectory: ArtifactStore.flutterRoot);
+
+    if (code != 0)
+      return code;
+
+    return await runCommandAndStreamOutput([sdkBinaryName('pub'), 'upgrade']);
+  }
+}