Do not allow native syntax on all libraries.

dart/frog/leg/compiler.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 WorkItem {
   final Element element;
   TreeElements resolutionTree;
   Function run;
   Map<int, BailoutInfo> bailouts = null;
   bool allowSpeculativeOptimization = true;
@@ skipping 241 matching lines @@
             }
           }
         }
       }
     }
   }
 
   void runCompiler(Script script) {
     scanBuiltinLibraries();
     mainApp = new LibraryElement(script);
-    native.checkNativeSupport(this, mainApp, null);
+    native.maybeEnableNative(this, mainApp, null);
 
     Element element;
     withCurrentElement(mainApp, () {
         scanner.scan(currentElement);
         element = mainApp.find(MAIN);
         if (element === null) cancel('Could not find $MAIN');
       });
     worklist.add(new WorkItem.toCompile(element));
     do {
       while (!worklist.isEmpty()) {
@@ skipping 151 matching lines @@
 
 // TODO(ahe): Remove when the VM supports implicit interfaces.
 class LTracer implements Tracer {
   const LTracer();
   final bool enabled = false;
   void traceGraph(String name, var graph) {
   }
   void close() {
   }
 }