Ensure locations are always provided, but don't store them yet.

pkg/front_end/lib/src/fasta/builder/mixin_application_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.mixin_application_builder;
 
 import '../errors.dart' show
     internalError;
 
 import 'builder.dart' show
     TypeBuilder,
     TypeDeclarationBuilder;
 
 import 'scope.dart' show
     Scope;
 
 abstract class MixinApplicationBuilder<T extends TypeBuilder>
     extends TypeBuilder {
   final T supertype;
   final List<T> mixins;
 
-  MixinApplicationBuilder(this.supertype, this.mixins);
+  MixinApplicationBuilder(this.supertype, this.mixins, int charOffset,
+      Uri fileUri)
+      : super(charOffset, fileUri);
 
   String get name => null;
 
   void resolveIn(Scope scope) {
     supertype.resolveIn(scope);
     for (T t in mixins) {
       t.resolveIn(scope);
     }
   }
 
   void bind(TypeDeclarationBuilder builder) {
     internalError("Internal error: can't bind a mixin application.");
   }
 
   String get debugName => "MixinApplicationBuilder";
 
   StringBuffer printOn(StringBuffer buffer) {
     buffer.write(supertype);
     buffer.write(" with ");
     bool first = true;
     for (T t in mixins) {
       if (!first) buffer.write(", ");
       first = false;
       t.printOn(buffer);
     }
     return buffer;
   }
 }