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

pkg/dart2js_incremental/lib/caching_compiler.dart

 // Copyright (c) 2013, 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 dart2js_incremental;
 
 /// Do not call this method directly. It will be made private.
 // TODO(ahe): Make this method private.
 Future<Compiler> reuseCompiler(
     {DiagnosticHandler diagnosticHandler,
@@ skipping 105 matching lines @@
     backend.emitter.typeTestRegistry
         ..checkedClasses = null
         ..checkedFunctionTypes = null
         ..rtiNeededClasses.clear()
         ..cachedClassesUsingTypeVariableTests = null;
 
     backend.emitter.oldEmitter.interceptorEmitter
         ..interceptorInvocationNames.clear();
 
     backend.emitter.nativeEmitter
-        ..nativeClasses.clear()
+        ..hasNativeClasses = false
         ..nativeMethods.clear();
 
     backend.emitter.readTypeVariables.clear();
 
     backend.emitter.oldEmitter
         ..outputBuffers.clear()
         ..isolateProperties = null
         ..classesCollector = null
         ..nativeClasses.clear()
         ..mangledFieldNames.clear()
@@ skipping 59 matching lines @@
   final Map<String, String> output = new Map<String, String>();
 
   EventSink<String> call(String name, String extension) {
     return new StringEventSink((String data) {
       output['$name.$extension'] = data;
     });
   }
 
   String operator[] (String key) => output[key];
 }