Fixing JS size regression from Blink roll.

tools/dom/scripts/dartmetadata.py

Index: tools/dom/scripts/dartmetadata.py
diff --git a/tools/dom/scripts/dartmetadata.py b/tools/dom/scripts/dartmetadata.py
index 8b6063cd3f724fea6fc348e012d896e08facfcb7..b59f55247b5195a6b11d21269be4aabfd9983bdc 100644
--- a/tools/dom/scripts/dartmetadata.py
+++ b/tools/dom/scripts/dartmetadata.py
@@ -202,13 +202,18 @@ _dart2js_annotations = monitored.Dict('dartmetadata._dart2js_annotations', {
       "@Returns('int|String|Null')",
     ],
 
-    'MessageEvent.ports': ["@Creates('JSExtendableArray')"],
-
     'MessageEvent.data': [
       "@annotation_Creates_SerializedScriptValue",
       "@annotation_Returns_SerializedScriptValue",
     ],
 
+    'MessageEvent.ports': ["@Creates('JSExtendableArray')"],
+
+    'MessageEvent.source': [
+      "@Creates('Null')",
+      "@Returns('EventTarget|=Object')",
+    ],
+
     'Metadata.modificationTime': [
       "@Creates('Null')", # JS date object.
     ],