Do not allow native syntax on all libraries.

dart/frog/leg/scanner/scanner_task.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 ScannerTask extends CompilerTask {
   ScannerTask(Compiler compiler) : super(compiler);
   String get name() => 'Scanner';
 
   void scan(CompilationUnitElement compilationUnit) {
     measure(() {
       if (compilationUnit.kind === ElementKind.LIBRARY) {
         compiler.log("scanning library ${compilationUnit.script.name}");
       }
       scanElements(compilationUnit);
       if (compilationUnit.kind === ElementKind.LIBRARY) {
         processScriptTags(compilationUnit);
       }
-      native.processNativeClasses(compiler, compilationUnit);
+      compiler.nativeHandler.processNativeClasses(compilationUnit);
     });
   }
 
   void processScriptTags(LibraryElement library) {
     LinkBuilder<ScriptTag> imports = new LinkBuilder<ScriptTag>();
     Uri cwd = new Uri(scheme: 'file', path: compiler.currentDirectory);
     Uri base = cwd.resolve(library.script.name.toString());
     for (ScriptTag tag in library.tags.reverse()) {
       SourceString argument = tag.argument.value.copyWithoutQuotes(1, 1);
       Uri resolved = base.resolve(argument.slowToString());
@@ skipping 56 matching lines @@
     compiler.dietParser.dietParse(compilationUnit, tokens);
   }
 
   LibraryElement loadLibrary(Uri uri, ScriptTag node) {
     bool newLibrary = false;
     LibraryElement library =
       compiler.universe.libraries.putIfAbsent(uri.toString(), () {
           newLibrary = true;
           Script script = compiler.readScript(uri, node);
           LibraryElement element = new LibraryElement(script);
-          native.checkNativeSupport(compiler, element, uri);
+          compiler.nativeHandler.checkNativeSupport(element, uri);
           return element;
         });
     if (newLibrary) {
       compiler.withCurrentElement(library, () => scan(library));
     }
     return library;
   }
 
   void importLibrary(LibraryElement library, LibraryElement imported,
                      LiteralString prefix) {
@@ skipping 15 matching lines @@
   final String name = 'Diet Parser';
 
   dietParse(CompilationUnitElement compilationUnit, Token tokens) {
     measure(() {
       ElementListener listener = new ElementListener(compiler, compilationUnit);
       PartialParser parser = new PartialParser(listener);
       parser.parseUnit(tokens);
     });
   }
 }