Skip native clauses when parsing directives.

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

Index: pkg/front_end/test/src/incremental/mock_sdk.dart
diff --git a/pkg/front_end/test/src/incremental/mock_sdk.dart b/pkg/front_end/test/src/incremental/mock_sdk.dart
index a6ffbe4a39a70155797c59209215cdeb4f52c8b4..973456aa56c5260395b7553db380913c70af7ac0 100644
--- a/pkg/front_end/test/src/incremental/mock_sdk.dart
+++ b/pkg/front_end/test/src/incremental/mock_sdk.dart
@@ -84,14 +84,29 @@ Future _awaitHelper(
   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;