Add a Pub source that checks packages out from Git.

utils/pub/pub.dart

Index: utils/pub/pub.dart
diff --git a/utils/pub/pub.dart b/utils/pub/pub.dart
index d066a6c12d628613186c80029d1ef4310350d254..be7e40bc1902053ee08ea28122d16e5abfdcf61a 100644
--- a/utils/pub/pub.dart
+++ b/utils/pub/pub.dart
@@ -20,7 +20,9 @@
 #source('command_version.dart');
 #source('package.dart');
 #source('source.dart');
+#source('source_registry.dart');
 #source('sdk_source.dart');
+#source('git_source.dart');
 
 main() {
   final args = new Options().arguments;
@@ -70,10 +72,10 @@ main() {
     }
   }
 
-  // TODO(rnystrom): Do we want this to be global?
   final cache = new SystemCache(cacheDir);
-
-  Source.defaultSource = new SdkSource(sdkDir);
+  cache.sources.register(new SdkSource(sdkDir));
+  cache.sources.register(new GitSource());
+  cache.sources.setDefault('sdk');
 
   // Select the command.
   final command = commands[args[0]];
@@ -137,7 +139,7 @@ class PubCommand {
     // TODO(rnystrom): Will eventually need better logic to walk up
     // subdirectories until we hit one that looks package-like. For now, just
     // assume the cwd is it.
-    Package.load(workingDir).then((pkg) {
+    Package.load(workingDir, cache.sources).then((pkg) {
       packagesDir = new PackagesDir(pkg, cache);
       onRun();
     });