Mark native methods as external.

lib/custom_compiler_options.dart

 // Copyright (c) 2016, 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.md file.
 
 library rasta.custom_compiler_options;
 
 import 'package:compiler/src/options.dart' show
     CompilerOptions;
 
 import 'package:compiler/compiler.dart' show
@@ skipping 46 matching lines @@
   @override bool get hasBuildId => buildId != _UNDETERMINED_BUILD_ID;
 
   @override Uri get entryPoint => _unsupported;
 
   @override Uri get packageRoot => null;
 
   @override PackagesDiscoveryProvider get packagesDiscoveryProvider => null;
 
   @override bool get allowMockCompilation => false;
 
-  @override bool get allowNativeExtensions => true;
+  @override bool get allowNativeExtensions => false;
 
   @override Uri get deferredMapUri => null;
 
   @override bool get disableInlining => true;
 
   @override bool get disableTypeInference => true;
 
   @override bool get dumpInfo => false;
 
   @override bool get enableAssertMessage => false;
@@ skipping 115 matching lines @@
         packagesDiscoveryProvider: null,
         environment: environment,
         options: arguments);
 
     return copy(options, base: base,
         platformConfigUri: targetSpecification.platformConfig);
   }
 }
 
 get _unsupported => throw "not supported";