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

dart/sdk/lib/_internal/compiler/implementation/js/nodes.dart

Index: dart/sdk/lib/_internal/compiler/implementation/js/nodes.dart
diff --git a/dart/sdk/lib/_internal/compiler/implementation/js/nodes.dart b/dart/sdk/lib/_internal/compiler/implementation/js/nodes.dart
index ec1469c9a815ae3635b52702d42fb8f60ee0b52e..f206c0e68d850896c56762ce999b6331b34390dc 100644
--- a/dart/sdk/lib/_internal/compiler/implementation/js/nodes.dart
+++ b/dart/sdk/lib/_internal/compiler/implementation/js/nodes.dart
@@ -149,6 +149,8 @@ abstract class Node {
 
   accept(NodeVisitor visitor);
   void visitChildren(NodeVisitor visitor);
+
+  VariableUse asVariableUse() => null;
 }
 
 class Program extends Node {
@@ -657,6 +659,8 @@ class VariableUse extends VariableReference {
   VariableUse(String name) : super(name);
 
   accept(NodeVisitor visitor) => visitor.visitVariableUse(this);
+
+  VariableUse asVariableUse() => this;
 }
 
 class VariableDeclaration extends VariableReference {