Skip native clauses when parsing directives.

pkg/front_end/test/src/incremental/mock_sdk.dart

 // Copyright (c) 2017, 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.
 
 import 'package:front_end/memory_file_system.dart';
 
 /// Create SDK libraries which are used by Fasta to perform kernel generation.
 /// Return the mapping from the simple names of these library to the URIs
 /// in the given [fileSystem].  The root of the SDK is `file:///sdk`.
 Map<String, Uri> createSdkFiles(MemoryFileSystem fileSystem) {
@@ skipping 66 matching lines @@
 Function _asyncErrorWrapperHelper(continuation) {}
 Future _awaitHelper(
     object, Function thenCallback, Function errorCallback, var awaiter) {}
 ''');
 
   addSdkLibrary('collection', 'library dart.collection;');
   addSdkLibrary('convert', 'library dart.convert;');
   addSdkLibrary('developer', 'library dart.developer;');
   addSdkLibrary('io', 'library dart.io;');
   addSdkLibrary('isolate', 'library dart.isolate;');
-  addSdkLibrary('math', 'library dart.math;');
+  addSdkLibrary(
+      'math',
+      '''
+library dart.math;
+double sin(num radians) => _sin(radians.toDouble());
+double _sin(double x) native "Math_sin";
+''');
   addSdkLibrary('mirrors', 'library dart.mirrors;');
   addSdkLibrary('nativewrappers', 'library dart.nativewrappers;');
   addSdkLibrary('profiler', 'library dart.profiler;');
   addSdkLibrary('typed_data', 'library dart.typed_data;');
   addSdkLibrary('vmservice_io', 'library dart.vmservice_io;');
   addSdkLibrary('_builtin', 'library dart._builtin;');
-  addSdkLibrary('_internal', 'library dart._internal; class Symbol {}');
+  addSdkLibrary(
+      '_internal',
+      '''
+library dart._internal;
+class Symbol {}
+class ExternalName {
+  final String name;
+  const ExternalName(this.name);
+}
+''');
   addSdkLibrary('_vmservice', 'library dart._vmservice;');
 
   return dartLibraries;
 }