Add an SDK source.

lib/src/flutter.dart

 // Copyright (c) 2016, the Dart project authors.  Please see the AUTHORS file
 // for details. 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:io';
 
 import 'package:path/path.dart' as p;
 import 'package:pub_semver/pub_semver.dart';
 
 import 'io.dart';
 
 /// Whether the Flutter SDK is available.
 final bool isAvailable = Platform.environment.containsKey("FLUTTER_ROOT");
 
 /// The path to the root directory of the Flutter SDK.
 final String rootDirectory = Platform.environment["FLUTTER_ROOT"];
 
 /// The Flutter SDK's version number, or `null` if the Flutter SDK is
 /// unavailable.
 final version = () {
   if (!isAvailable) return null;
 
   return new Version.parse(
       readTextFile(p.join(rootDirectory, "version")).trim());
 }();
+
+/// Returns the path to the package [name] within Flutter.
+String packagePath(String name) {
+  if (!isAvailable) throw new StateError("Flutter is not available.");
+
+  return p.join(rootDirectory, 'packages', name);
+}