Add accessor for getting target-specific constructor.

pkg/front_end/lib/src/fasta/loader.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.loader;
 
 import 'dart:async' show
     Future;
 
 import 'dart:collection' show
     Queue;
 
 import 'ast_kind.dart' show
     AstKind;
 
 import 'builder/builder.dart' show
+    Builder,
     LibraryBuilder;
 
 import 'errors.dart' show
     InputError,
     firstSourceUri;
 
 import 'target_implementation.dart' show
     TargetImplementation;
 
 import 'ticker.dart' show
@@ skipping 113 matching lines @@
 
   List<InputError> collectCompileTimeErrors() {
     List<InputError> errors = <InputError>[];
     for (LibraryBuilder library in builders.values) {
       if (library.loader == this) {
         errors.addAll(library.compileTimeErrors);
       }
     }
     return errors;
   }
+
+  Builder getCompileTimeError() => target.getCompileTimeError(this);
 }
 
 String format(double d, int fractionDigits, int width) {
   return d.toStringAsFixed(fractionDigits).padLeft(width);
 }