Add helpers library to dart2js.

sdk/lib/_internal/compiler/implementation/warnings.dart

Index: sdk/lib/_internal/compiler/implementation/warnings.dart
diff --git a/sdk/lib/_internal/compiler/implementation/warnings.dart b/sdk/lib/_internal/compiler/implementation/warnings.dart
index f50505a71f80b94f1987a0d4720a4abe098dd883..c9a98b5cdc6107964f2938c14aba9399434122ef 100644
--- a/sdk/lib/_internal/compiler/implementation/warnings.dart
+++ b/sdk/lib/_internal/compiler/implementation/warnings.dart
@@ -1474,10 +1474,20 @@ main() {}
           examples: const ["do() {} main() {}"]);
 
   static const MessageKind UNUSED_METHOD = const MessageKind(
-      "The method '#{method_name}' is never called.",
+      "The method '#{name}' is never called.",
       howToFix: "Consider deleting it.",
       examples: const ["deadCode() {} main() {}"]);
 
+  static const MessageKind UNUSED_CLASS = const MessageKind(
+      "The class '#{name}' is never used.",
+      howToFix: "Consider deleting it.",
+      examples: const ["class DeadCode {} main() {}"]);
+
+  static const MessageKind UNUSED_TYPEDEF = const MessageKind(
+      "The typedef '#{name}' is never used.",
+      howToFix: "Consider deleting it.",
+      examples: const ["typedef DeadCode(); main() {}"]);
+
   static const MessageKind ABSTRACT_METHOD = const MessageKind(
       "The method '#{name}' has no implementation in "
       "class '#{class}'.",