Add an SDK source.

lib/src/source_registry.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 'source.dart';
 import 'source/git.dart';
 import 'source/hosted.dart';
 import 'source/path.dart';
+import 'source/sdk.dart';
 import 'source/unknown.dart';
 
 /// A class that keeps track of [Source]s used for getting packages.
 class SourceRegistry {
   /// The registered sources.
   ///
   /// This is initialized with the three built-in sources.
   final _sources = {
     "git": new GitSource(),
     "hosted": new HostedSource(),
-    "path": new PathSource()
+    "path": new PathSource(),
+    "sdk": new SdkSource()
   };
 
   /// The default source, which is used when no source is specified.
   ///
   /// This defaults to [hosted].
   Source get defaultSource => _default;
   Source _default;
 
   /// The registered sources, in name order.
   List<Source> get all {
     var sources = _sources.values.toList();
     sources.sort((a, b) => a.name.compareTo(b.name));
     return sources;
   }
 
   /// The built-in [GitSource].
   GitSource get git => _sources["git"] as GitSource;
 
   /// The built-in [HostedSource].
   HostedSource get hosted => _sources["hosted"] as HostedSource;
 
   /// The built-in [PathSource].
   PathSource get path => _sources["path"] as PathSource;
 
+  /// The built-in [SdkSource].
+  SdkSource get sdk => _sources["sdk"] as SdkSource;
+
   SourceRegistry() {
     _default = hosted;
   }
 
   /// Sets the default source.
   ///
   /// This takes a string, which must be the name of a registered source.
   void setDefault(String name) {
     if (!_sources.containsKey(name)) {
       throw new StateError('Default source $name is not in the registry');
@@ skipping 18 matching lines @@
   /// Returns the source named [name].
   ///
   /// Returns an [UnknownSource] if no source with that name has been
   /// registered. If [name] is null, returns the default source.
   Source operator[](String name) {
     if (name == null) return _default;
     if (_sources.containsKey(name)) return _sources[name];
     return new UnknownSource(name);
   }
 }