Emit defineClass function only if it is used.

lib/compiler/implementation/emitter.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.
 
 /**
  * A function element that represents a closure call. The signature is copied
  * from the given element.
  */
 class ClosureInvocationElement extends FunctionElement {
   ClosureInvocationElement(SourceString name,
@@ skipping 157 matching lines @@
   str += "}\\n";
   var newIsolate = new Function(str);
   newIsolate.prototype = isolatePrototype;
   isolatePrototype.constructor = newIsolate;
   newIsolate.${namer.ISOLATE_PROPERTIES} = isolateProperties;
   return newIsolate;
 }""";
   }
 
   void addDefineClassAndFinishClassFunctionsIfNecessary(StringBuffer buffer) {
-    String isolate = namer.ISOLATE;
-    buffer.add("$defineClassName = $defineClassFunction;\n");
-    buffer.add("$pendingClassesName = {};\n");
-    buffer.add("$finishClassesName = $finishClassesFunction;\n");
+    if (needsDefineClass) {
+      String isolate = namer.ISOLATE;
+      buffer.add("$defineClassName = $defineClassFunction;\n");
+      buffer.add("$pendingClassesName = {};\n");
+      buffer.add("$finishClassesName = $finishClassesFunction;\n");
+    }
   }
 
   void emitFinishIsolateConstructor(StringBuffer buffer) {
     String name = finishIsolateConstructorName;
     String value = finishIsolateConstructorFunction;
     buffer.add("$name = $value;\n");
   }
 
   void emitFinishIsolateConstructorInvocation(StringBuffer buffer) {
     String isolate = namer.ISOLATE;
@@ skipping 701 matching lines @@
       mainBuffer.add('function init() {\n');
       mainBuffer.add('  $isolateProperties = {};\n');
       addDefineClassAndFinishClassFunctionsIfNecessary(mainBuffer);
       emitFinishIsolateConstructor(mainBuffer);
       mainBuffer.add('}\n');
       compiler.assembledCode = mainBuffer.toString();
     });
     return compiler.assembledCode;
   }
 }