Implement isSubclassOf for dart2js source mirrors.

sdk/lib/_internal/compiler/implementation/mirrors/dart2js_type_mirrors.dart

Index: sdk/lib/_internal/compiler/implementation/mirrors/dart2js_type_mirrors.dart
diff --git a/sdk/lib/_internal/compiler/implementation/mirrors/dart2js_type_mirrors.dart b/sdk/lib/_internal/compiler/implementation/mirrors/dart2js_type_mirrors.dart
index 3dffd960bf47244846f4eae6838df35940ca03f7..af91ba4d6bec054e981eead09699795392895643 100644
--- a/sdk/lib/_internal/compiler/implementation/mirrors/dart2js_type_mirrors.dart
+++ b/sdk/lib/_internal/compiler/implementation/mirrors/dart2js_type_mirrors.dart
@@ -193,6 +193,14 @@ class Dart2JsInterfaceTypeMirror
     return null;
   }
 
+  bool isSubclassOf(Mirror other) {
+    if (other is Dart2JsTypeMirror) {
+      return _element.isSubclassOf(other._type.element);
+    } else {
+      throw new ArgumentError(other);
+    }
+  }
+
   ClassMirror get mixin {
     if (_element.isMixinApplication) {
       MixinApplicationElement mixinApplication = _element;
@@ -407,6 +415,8 @@ class Dart2JsFunctionTypeMirror extends Dart2JsTypeMirror
   }
 
   String toString() => 'Mirror on function type $_type';
+
+  bool isSubclassOf(ClassMirror other) => false;
 }
 
 class Dart2JsVoidMirror extends Dart2JsTypeMirror {