Do not allow native syntax on all libraries.

dart/frog/leg/scanner/scanner_task.dart

Index: dart/frog/leg/scanner/scanner_task.dart
===================================================================
--- dart/frog/leg/scanner/scanner_task.dart	(revision 4858)
+++ dart/frog/leg/scanner/scanner_task.dart	(working copy)
@@ -15,7 +15,7 @@
       if (compilationUnit.kind === ElementKind.LIBRARY) {
         processScriptTags(compilationUnit);
       }
-      native.processNativeClasses(compiler, compilationUnit);
+      compiler.nativeHandler.processNativeClasses(compilationUnit);
     });
   }
 
@@ -92,7 +92,7 @@
           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) {