dart2js: use bool to track if an app has native classes.

pkg/compiler/lib/src/js_backend/native_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.
 
 part of js_backend;
 
 class NativeEmitter {
 
   final Map<Element, ClassBuilder> cachedBuilders;
 
   final CodeEmitterTask emitterTask;
 
-  // Native classes found in the application.
-  Set<ClassElement> nativeClasses = new Set<ClassElement>();
+  // Whether the application contains native classes.
+  bool hasNativeClasses = false;
 
   // Caches the native subtypes of a native class.
   Map<ClassElement, List<ClassElement>> subtypes;
 
   // Caches the direct native subtypes of a native class.
   Map<ClassElement, List<ClassElement>> directSubtypes;
 
   // Caches the methods that have a native body.
   Set<FunctionElement> nativeMethods;
 
@@ skipping 246 matching lines @@
           .add(classElement);
       }
     }
     return map;
   }
 
   ClassBuilder generateNativeClass(ClassElement classElement) {
     // TODO(sra): Issue #13731- this is commented out as part of custom element
     // constructor work.
     //assert(!classElement.hasBackendMembers);
-    nativeClasses.add(classElement);
+    hasNativeClasses = true;
 
     ClassElement superclass = classElement.superclass;
     assert(superclass != null);
     // Fix superclass.  TODO(sra): make native classes inherit from Interceptor.
     assert(superclass != compiler.objectClass);
     if (superclass == compiler.objectClass) {
       superclass = backend.jsInterceptorClass;
     }
 
     String superName = backend.namer.getNameOfClass(superclass);
@@ skipping 148 matching lines @@
 
   void assembleCode(CodeOutput targetOutput) {
     List<jsAst.Property> objectProperties = <jsAst.Property>[];
 
     jsAst.Property addProperty(String name, jsAst.Expression value) {
       jsAst.Property prop = new jsAst.Property(js.string(name), value);
       objectProperties.add(prop);
       return prop;
     }
 
-    if (!nativeClasses.isEmpty) {
+    if (hasNativeClasses) {
       // If the native emitter has been asked to take care of the
       // noSuchMethod handlers, we do that now.
       if (handleNoSuchMethod) {
         emitterTask.oldEmitter.nsmEmitter.emitNoSuchMethodHandlers(addProperty);
       }
     }
 
     // If we have any properties to add to Object.prototype, we run
     // through them and add them using defineProperty.
     if (!objectProperties.isEmpty) {
       jsAst.Expression init = js(r'''
           (function(table) {
             for(var key in table)
               #(Object.prototype, key, table[key]);
            })(#)''',
           [ defPropFunction,
             new jsAst.ObjectInitializer(objectProperties)]);
 
       if (emitterTask.compiler.enableMinification) {
         targetOutput.add(';');
       }
       targetOutput.addBuffer(jsAst.prettyPrint(
           new jsAst.ExpressionStatement(init), compiler));
       targetOutput.add('\n');
     }
 
     targetOutput.add('\n');
   }
 }