Emit defineClass function only if it is used.

lib/compiler/implementation/emitter.dart

Index: lib/compiler/implementation/emitter.dart
diff --git a/lib/compiler/implementation/emitter.dart b/lib/compiler/implementation/emitter.dart
index 68a3d1a3a0077c8102d6fa94373805a8dedeaa9d..de895ee701b1e567693f535209fa0779c24990ab 100644
--- a/lib/compiler/implementation/emitter.dart
+++ b/lib/compiler/implementation/emitter.dart
@@ -175,10 +175,12 @@ function() {
   }
 
   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) {