Avoid crashing on function syntax.

pkg/front_end/lib/src/fasta/builder/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.builder;
 
 import '../errors.dart' show internalError;
 
 import '../messages.dart' show nit;
 
@@ skipping 36 matching lines @@
 export 'prefix_builder.dart' show PrefixBuilder;
 
 export 'invalid_type_builder.dart' show InvalidTypeBuilder;
 
 export 'mixed_accessor.dart' show MixedAccessor;
 
 export 'scope.dart' show AccessErrorBuilder;
 
 export 'dynamic_type_builder.dart' show DynamicTypeBuilder;
 
+export 'function_type_builder.dart' show FunctionTypeBuilder;
+
 import 'library_builder.dart' show LibraryBuilder;
 
 abstract class Builder {
   /// Used when multiple things with the same name are declared within the same
   /// parent. Only used for declarations, not for scopes.
   ///
   // TODO(ahe): Move to member builder or something. Then we can make
   // this a const class.
   Builder next;
 
@@ skipping 98 matching lines @@
 
   static Uri getUri(Builder builder) {
     if (builder == null) return internalError("Builder is null.");
     while (builder != null) {
       if (builder is LibraryBuilder) return builder.uri;
       builder = builder.parent;
     }
     return internalError("No library parent.");
   }
 }