Add a Pub source that checks packages out from Git.

utils/pub/utils.dart

Index: utils/pub/utils.dart
diff --git a/utils/pub/utils.dart b/utils/pub/utils.dart
index 39c86dfa274c5b42f403da69618b84c3f92a7bb7..d0c2f435d21de48cd5cefdfb5a116148f503931d 100644
--- a/utils/pub/utils.dart
+++ b/utils/pub/utils.dart
@@ -50,3 +50,14 @@ List flatten(List nested) {
   helper(nested);
   return result;
 }
+
+/**
+ * Asserts that [iter] contains only one element, and returns it.
+ */
+only(Iterable iter) {
+  var iterator = iter.iterator();
+  assert(iterator.hasNext());
+  var obj = iterator.next();
+  assert(!iterator.hasNext());
+  return obj;
+}