Remove all warnings from dart2js and add a test to prevent introduction of new.

dart/sdk/lib/_internal/compiler/implementation/js_backend/native_emitter.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.
 
 part of js_backend;
 
 class NativeEmitter {
 
   CodeEmitterTask emitter;
   CodeBuffer nativeBuffer;
@@ skipping 381 matching lines @@
         for (final ClassElement subclass in getDirectSubclasses(cls)) {
           ClassElement tag = subclass;
           js.Expression existing = tagDefns[tag];
           if (existing == null) {
             // [subclass] is still within the subtree between dispatch classes.
             subtags.add(toNativeTag(tag));
             walk(subclass);
           } else {
             // [subclass] is one of the preorderDispatchClasses, so CSE this
             // reference with the previous reference.
-            if (existing is js.VariableUse &&
-                varDefns.containsKey(existing.name)) {
+            js.VariableUse use = existing.asVariableUse();
+            if (use != null && varDefns.containsKey(use.name)) {
               // We end up here if the subclasses have a DAG structure.  We
               // don't have DAGs yet, but if the dispatch is used for mixins
               // that will be a possibility.
               // Re-use the previously created temporary variable.
-              expressions.add(new js.VariableUse(existing.name));
+              expressions.add(new js.VariableUse(use.name));
             } else {
               String varName = 'v${varNames.length}_${tag.name.slowToString()}';
               varNames.add(varName);
               varDefns[varName] = existing;
               tagDefns[tag] = new js.VariableUse(varName);
               expressions.add(new js.VariableUse(varName));
             }
           }
         }
       }
@@ skipping 147 matching lines @@
         if (!first) targetBuffer.add(",\n");
         targetBuffer.add(" $name: $function");
         first = false;
       });
       targetBuffer.add("\n});\n\n");
     }
     targetBuffer.add(nativeBuffer);
     targetBuffer.add('\n');
   }
 }