Change the calling conventions for handleUnrecoverableError.

pkg/front_end/lib/src/fasta/source/outline_builder.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 file.
 
 library fasta.outline_builder;
 
 import 'package:kernel/ast.dart' show AsyncMarker, ProcedureKind;
 
 import '../parser/parser.dart' show FormalParameterType, optional;
 
@@ skipping 671 matching lines @@
     debugEvent("Modifiers");
     push(popList(count) ?? NullValue.Modifiers);
   }
 
   @override
   Token handleUnrecoverableError(Token token, ErrorKind kind, Map arguments) {
     if (isDartLibrary && kind == ErrorKind.ExpectedBlockToSkip) {
       Token recover = skipNativeClause(token);
       if (recover != null) {
         nativeMethodName = unescapeString(token.next.value);
-        return recover;
+        return newSyntheticToken(recover);
       }
     }
     return super.handleUnrecoverableError(token, kind, arguments);
   }
 
   @override
   Link<Token> handleMemberName(Link<Token> identifiers) {
     if (!isDartLibrary || identifiers.isEmpty) return identifiers;
     return removeNativeClause(identifiers);
   }
 
   @override
   void debugEvent(String name) {
     // printEvent(name);
   }
 }