Add a Pub source that checks packages out from Git.

utils/pub/utils.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.
 
 /**
  * Generic utility functions. Stuff that should possibly be in core.
  */
 #library('pub_utils');
 
 // TODO(rnystrom): Move into String?
@@ skipping 32 matching lines @@
       if (element is List) {
         helper(element);
       } else {
         result.add(element);
       }
     }
   }
   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;
+}