Clean up code that locates SDK and SDK version.

utils/pub/root_source.dart

 // Copyright (c) 2012, 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.
 
 library root_source;
 
+import 'dart:async';
 import 'package.dart';
 import 'pubspec.dart';
 import 'source.dart';
 
 /// A source used only for the root package when doing version resolution. It
 /// contains only the root package and is unable to install packages.
 ///
 /// This source cannot be referenced from a pubspec.
 class RootSource extends Source {
   final String name = "root";
   final bool shouldCache = false;
   final Package package;
 
   RootSource(this.package);
 
   Future<Pubspec> describe(PackageId id) {
     return new Future<Pubspec>.immediate(package.pubspec);
   }
 
   Future<bool> install(PackageId id, String destPath) {
     throw new UnsupportedError("Can't install from a root source.");
   }
 }