Make undefined and null equivalent.

lib/compiler/implementation/compile_time_constants.dart

Index: lib/compiler/implementation/compile_time_constants.dart
diff --git a/lib/compiler/implementation/compile_time_constants.dart b/lib/compiler/implementation/compile_time_constants.dart
index ae7fe1d2eaa8afa7caaa26c67114e44e162bb038..86cfe4fed3cfdd40a29652c1cf2a5e3ad830248c 100644
--- a/lib/compiler/implementation/compile_time_constants.dart
+++ b/lib/compiler/implementation/compile_time_constants.dart
@@ -56,13 +56,16 @@ class PrimitiveConstant extends Constant {
 }
 
 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.