Fix warnings in dart2js.

sdk/lib/_internal/compiler/implementation/mirrors/dart2js_mirror.dart

 // Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 library mirrors_dart2js;
 
 import 'dart:async';
 
 import '../../compiler.dart' as api;
 import '../elements/elements.dart';
@@ skipping 474 matching lines @@
     return new AsFilteredImmutableMap<String, MemberMirror, MethodMirror>(
         _members,
         (MemberMirror member) => member is MethodMirror ? member : null);
   }
 
   Map<String, MethodMirror> get getters {
     _ensureMembers();
     return new AsFilteredImmutableMap<String, MemberMirror, MethodMirror>(
         _members,
         (MemberMirror member) =>
-            member is MethodMirror && (member as MethodMirror).isGetter ?
-                member : null);
+            member is MethodMirror && member.isGetter ? member : null);
   }
 
   Map<String, MethodMirror> get setters {
     _ensureMembers();
     return new AsFilteredImmutableMap<String, MemberMirror, MethodMirror>(
         _members,
         (MemberMirror member) =>
-            member is MethodMirror && (member as MethodMirror).isSetter ?
-                member : null);
+            member is MethodMirror && member.isSetter ? member : null);
   }
 
   Map<String, VariableMirror> get variables {
     _ensureMembers();
     return new AsFilteredImmutableMap<String, MemberMirror, VariableMirror>(
         _members,
         (MemberMirror member) => member is VariableMirror ? member : null);
   }
 }
 
@@ skipping 1233 matching lines @@
  *
  * All API in this class is experimental.
  */
 class BackDoor {
   /// Return the compilation units comprising [library].
   static List<Mirror> compilationUnitsOf(Dart2JsLibraryMirror library) {
     return library._library.compilationUnits.toList().map(
         (cu) => new Dart2JsCompilationUnitMirror(cu, library)).toList();
   }
 }