Make mappedBy lazy.

sdk/lib/html/src/CssClassSet.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.
 
 abstract class CssClassSet implements Set<String> {
 
   String toString() {
     return Strings.join(new List.from(readClasses()), ' ');
   }
 
@@ skipping 22 matching lines @@
 
   // interface Iterable - BEGIN
   Iterator<String> iterator() => readClasses().iterator();
   // interface Iterable - END
 
   // interface Collection - BEGIN
   void forEach(void f(String element)) {
     readClasses().forEach(f);
   }
 
-  Collection mappedBy(f(String element)) => readClasses().mappedBy(f);
+  Iterable mappedBy(f(String element)) => readClasses().mappedBy(f);
 
   Collection<String> where(bool f(String element)) => readClasses().where(f);
 
   bool every(bool f(String element)) => readClasses().every(f);
 
   bool some(bool f(String element)) => readClasses().some(f);
 
   bool get isEmpty => readClasses().isEmpty;
 
   int get length =>readClasses().length;
@@ skipping 60 matching lines @@
    */
   Set<String> readClasses();
 
   /**
    * Join all the elements of a set into one string and write
    * back to the element.
    * This is intended to be overridden by specific implementations.
    */
   void writeClasses(Set<String> s);
 }