Add an SDK source.

lib/src/source_registry.dart

Index: lib/src/source_registry.dart
diff --git a/lib/src/source_registry.dart b/lib/src/source_registry.dart
index be49746dc0b3eb824342b7c2aaaef6e74fdf1d3c..9fffff49b2e92be8477ad194173511296b400868 100644
--- a/lib/src/source_registry.dart
+++ b/lib/src/source_registry.dart
@@ -6,6 +6,7 @@ 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.
@@ -16,7 +17,8 @@ class SourceRegistry {
   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.
@@ -41,6 +43,9 @@ class SourceRegistry {
   /// 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;
   }