Make undefined and null equivalent.

lib/compiler/implementation/compile_time_constants.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.
 
 class Constant implements Hashable {
   const Constant();
 
   bool isNull() => false;
   bool isBool() => false;
   bool isTrue() => false;
@@ skipping 38 matching lines @@
   // Primitive constants don't have dependencies.
   List<Constant> getDependencies() => const <Constant>[];
   abstract DartString toDartString();
 
   void _writeCanonicalizedJsCode(StringBuffer buffer, ConstantHandler handler) {
     _writeJsCode(buffer, handler);
   }
 }
 
 class NullConstant extends PrimitiveConstant {
+  /** The value a Dart null is compiled to in JavaScript. */
+  static final String JsNull = "null";
+
   factory NullConstant() => const NullConstant._internal();
   const NullConstant._internal();
   bool isNull() => true;
   get value() => null;
 
   void _writeJsCode(StringBuffer buffer, ConstantHandler handler) {
-    buffer.add("(void 0)");
+    buffer.add(JsNull);
   }
 
   // The magic constant has no meaning. It is just a random value.
   int hashCode() => 785965825;
   DartString toDartString() => const LiteralDartString("null");
 }
 
 class NumConstant extends PrimitiveConstant {
   abstract num get value();
   const NumConstant();
@@ skipping 1087 matching lines @@
       Constant fieldValue = fieldValues[field];
       if (fieldValue === null) {
         // Use the default value.
         fieldValue = compiler.compileVariable(field);
       }
       jsNewArguments.add(fieldValue);
     });
     return jsNewArguments;
   }
 }