Fix custom elements in startup emitter

pkg/compiler/lib/src/js_emitter/startup_emitter/fragment_emitter.dart

 // Copyright (c) 2015, 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.
 
 part of dart2js.js_emitter.startup_emitter.model_emitter;
 
 /// The name of the property that stores the tear-off getter on a static
 /// function.
 ///
 /// This property is only used when isolates are used.
@@ skipping 1315 matching lines @@
         function (s) {
           var o = {};
           o[s] = 1;
           return Object.keys(convertToFastObject(o))[0];
         }""",
               [])));
     }
 
     Map<String, js.Expression> interceptorsByTag = <String, js.Expression>{};
     Map<String, js.Expression> leafTags = <String, js.Expression>{};
-    js.Statement subclassAssignment = new js.EmptyStatement();
+    List<js.Statement> subclassAssignments = <js.Statement>[];
 
     for (Library library in fragment.libraries) {
       for (Class cls in library.classes) {
         if (cls.nativeLeafTags != null) {
           for (String tag in cls.nativeLeafTags) {
             interceptorsByTag[tag] = classReference(cls);
             leafTags[tag] = new js.LiteralBool(true);
           }
         }
         if (cls.nativeNonLeafTags != null) {
           for (String tag in cls.nativeNonLeafTags) {
             interceptorsByTag[tag] = classReference(cls);
             leafTags[tag] = new js.LiteralBool(false);
           }
           if (cls.nativeExtensions != null) {
             List<Class> subclasses = cls.nativeExtensions;
-            js.Expression value = js.string(cls.nativeNonLeafTags[0]);
+            js.Expression base = js.string(cls.nativeNonLeafTags[0]);
+
             for (Class subclass in subclasses) {
-              value = js.js('#.# = #', [
+              subclassAssignments.add(js.js.statement('#.# = #;', [
                 classReference(subclass),
                 NATIVE_SUPERCLASS_TAG_NAME,
-                js.string(cls.nativeNonLeafTags[0])
-              ]);
+                base
+              ]));
             }
-            subclassAssignment = new js.ExpressionStatement(value);
           }
         }
       }
     }
     statements.add(js.js.statement("setOrUpdateInterceptorsByTag(#);",
         js.objectLiteral(interceptorsByTag)));
     statements.add(
         js.js.statement("setOrUpdateLeafTags(#);", js.objectLiteral(leafTags)));
-    statements.add(subclassAssignment);
+    statements.addAll(subclassAssignments);
 
     return new js.Block(statements);
   }
 }