Add runtimeType() to Object which returns canonicalized instances of Type.

lib/compiler/implementation/lib/js_helper.dart

Index: lib/compiler/implementation/lib/js_helper.dart
diff --git a/lib/compiler/implementation/lib/js_helper.dart b/lib/compiler/implementation/lib/js_helper.dart
index 8d1f3013035ed67bb9683086263fe286aa45f756..6332b1f12f6ef3eb5192ece8bbcce1b0a94db168 100644
--- a/lib/compiler/implementation/lib/js_helper.dart
+++ b/lib/compiler/implementation/lib/js_helper.dart
@@ -1289,3 +1289,23 @@ void throwNoSuchMethod(obj, name, arguments) {
 void throwCyclicInit(String staticName) {
   throw new RuntimeError("Cyclic initialization for static $staticName");
 }
+
+class TypeImpl implements Type {
+  final String typeName;
+  TypeImpl(this.typeName);
+  toString() => typeName;
+}
+
+Type getOrCreateCachedRuntimeType(String key) {
+  Type runtimeType = JS('Object', @'$.runtimeTypeCache[#]', key);
+  if (runtimeType == null) {
+    runtimeType = new TypeImpl(key);
+    JS('void', @'$.runtimeTypeCache[#] = #', key, runtimeType);
+  }
+  return runtimeType;
+}
+
+String getRuntimeTypeString(var object) {
+  var typeInfo = JS('Object', @'#.builtin$typeInfo', object);
+  return JS('String', @'#.runtimeType', typeInfo);
+}