Uses a HashMap to complete deferred library loads.

runtime/lib/lib_prefix.dart

Index: runtime/lib/lib_prefix.dart
diff --git a/runtime/lib/lib_prefix.dart b/runtime/lib/lib_prefix.dart
index fb9fe815400e55c5655e129b7b9d6c2b25e042ae..55b47b38bbd846bb950fe9e9675802d19ffd9659 100644
--- a/runtime/lib/lib_prefix.dart
+++ b/runtime/lib/lib_prefix.dart
@@ -3,6 +3,7 @@
 // BSD-style license that can be found in the LICENSE file.
 
 import "dart:async";
+import "dart:collection";
 import "dart:isolate";
 
 // This type corresponds to the VM-internal class LibraryPrefix.
@@ -39,7 +40,7 @@ class _LibraryPrefix {
   }
 }
 
-var _outstandingLoadRequests = new Map<_LibraryPrefix, Completer>();
+var _outstandingLoadRequests = new HashMap<_LibraryPrefix, Completer>();
 
 
 // Called from the VM when all outstanding load requests have