Interpose an Expando cache on reflectClass().

runtime/lib/mirrors_impl.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.
 
 // VM-specific implementation of the dart:mirrors library.
 
 // These values are allowed to be passed directly over the wire.
 bool _isSimpleValue(var value) {
   return (value == null || value is num || value is String || value is bool);
 }
@@ skipping 1102 matching lines @@
 
   // Creates a new local mirror for some Object.
   static InstanceMirror reflect(Object reflectee) {
     return makeLocalInstanceMirror(reflectee);
   }
 
   // Creates a new local ClassMirror.
   static ClassMirror makeLocalClassMirror(Type key)
       native "Mirrors_makeLocalClassMirror";
 
+  static Expando<ClassMirror> _classMirrorCache = new Expando("ClassMirror");
   static ClassMirror reflectClass(Type key) {
-    return makeLocalClassMirror(key);
+    var classMirror = _classMirrorCache[key];
+    if (classMirror == null) {
+      classMirror = makeLocalClassMirror(key);
+      _classMirrorCache[key] = classMirror;
+    }
+    return classMirror;
   }
 }