Optimize the message queue for many active ports with few messages.

runtime/lib/isolate_patch.dart

Index: runtime/lib/isolate_patch.dart
diff --git a/runtime/lib/isolate_patch.dart b/runtime/lib/isolate_patch.dart
index d2938255f019cf30802d60799759ee0fd5d7613c..c359783109598bdd471999919f4a53d5929e01ab 100644
--- a/runtime/lib/isolate_patch.dart
+++ b/runtime/lib/isolate_patch.dart
@@ -35,6 +35,7 @@ class _ReceivePortImpl implements ReceivePort {
     }
     return new _ReceivePortImpl._internal(id);
   }
+
   _ReceivePortImpl._internal(int id) : _id = id {
     if (_portMap == null) {
       _portMap = new Map();
@@ -42,10 +43,15 @@ class _ReceivePortImpl implements ReceivePort {
     _portMap[id] = this;
   }
 
-  // Called from the VM to dispatch to the handler.
-  static void _handleMessage(int id, int replyId, var message) {
+  // Called from the VM to retrieve the ReceivePort for a message.
+  static _ReceivePortImpl _lookupReceivePort(int id) {
     assert(_portMap != null);
-    ReceivePort port = _portMap[id];
+    return _portMap[id];
+  }
+
+  // Called from the VM to dispatch to the handler.
+  static void _handleMessage(_ReceivePortImpl port, int replyId, var message) {
+    assert(port != null);
     SendPort replyTo = (replyId == 0) ? null : new _SendPortImpl(replyId);
     (port._onMessage)(message, replyTo);
   }