Add an SDK source.

lib/src/system_cache.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.
 
 import 'dart:io';
 
 import 'package:path/path.dart' as p;
 
 import 'io.dart';
 import 'io.dart' as io show createTempDir;
 import 'log.dart' as log;
 import 'package.dart';
 import 'source/cached.dart';
 import 'source/git.dart';
 import 'source/hosted.dart';
 import 'source/path.dart';
+import 'source/sdk.dart';
 import 'source/unknown.dart';
 import 'source.dart';
 import 'source_registry.dart';
 
 /// The system-wide cache of downloaded packages.
 ///
 /// This cache contains all packages that are downloaded from the internet.
 /// Packages that are available locally (e.g. path dependencies) don't use this
 /// cache.
 class SystemCache {
@@ skipping 25 matching lines @@
   /// The built-in Git source bound to this cache.
   BoundGitSource get git => _boundSources[sources.git] as BoundGitSource;
 
   /// The built-in hosted source bound to this cache.
   BoundHostedSource get hosted =>
       _boundSources[sources.hosted] as BoundHostedSource;
 
   /// The built-in path source bound to this cache.
   BoundPathSource get path => _boundSources[sources.path] as BoundPathSource;
 
+  /// The built-in SDK source bound to this cache.
+  BoundSdkSource get sdk => _boundSources[sources.sdk] as BoundSdkSource;
+
   /// The default source bound to this cache.
   BoundSource get defaultSource => source(sources[null]);
 
   /// Creates a system cache and registers all sources in [sources].
   ///
   /// If [isOffline] is `true`, then the offline hosted source will be used.
   /// Defaults to `false`.
   SystemCache({String rootDir, bool isOffline: false})
       : rootDir = rootDir == null ? SystemCache.defaultDir : rootDir {
     for (var source in sources.all) {
@@ skipping 42 matching lines @@
     var temp = ensureDir(tempDir);
     return io.createTempDir(temp, 'dir');
   }
 
   /// Deletes the system cache's internal temp directory.
   void deleteTempDir() {
     log.fine('Clean up system cache temp directory $tempDir.');
     if (dirExists(tempDir)) deleteEntry(tempDir);
   }
 }