Fix broken unparser test in checked mode.

lib/compiler/implementation/dart_backend/backend.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.
 
 class BailoutException {
   final String reason;
 
   const BailoutException(this.reason);
 }
 
@@ skipping 23 matching lines @@
                             Collection<LibraryElement> libraries) {
   }
 
   /**
    * Adds given class element with its member element to resolved classes
    * collections.
    */
   void addMemberToClass(Element element, ClassElement classElement) {
     // ${element} should have ${classElement} as enclosing.
     assert(element.enclosingElement == classElement);
-    List<Element> resolvedElementsInClass =
-        resolvedClassMembers.putIfAbsent(classElement, () => <Element>[]);
+    Set<Element> resolvedElementsInClass = resolvedClassMembers.putIfAbsent(
+        classElement, () => new Set<Element>());
     resolvedElementsInClass.add(element);
   }
 
   /**
    * Outputs given class element with given inner elements to a string buffer.
    */
-  void outputClass(ClassElement classElement, List<Element> innerElements,
+  void outputClass(ClassElement classElement, Set<Element> innerElements,
       StringBuffer sb) {
     // TODO(smok): Very soon properly print out correct class declaration with
     // extends, implements, etc.
     sb.add('class ');
     sb.add(classElement.name.slowToString());
     sb.add('{');
     innerElements.forEach((element) {
       // TODO(smok): Filter out default constructors here.
       sb.add(element.parseNode(compiler).unparse());
     });
@@ skipping 64 matching lines @@
       compiler.assembledCode = '''
 main() {
   final bailout_reason = "${e.reason}";
 }
 ''';
     }
   }
 
   log(String message) => compiler.log('[DartBackend] $message');
 }