Add accessor for getting target-specific constructor.

pkg/front_end/lib/src/fasta/target_implementation.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.target_implementation;
 
 import 'builder/builder.dart' show
+    Builder,
     ClassBuilder,
     LibraryBuilder;
 
+import 'loader.dart' show
+    Loader;
+
 import 'target.dart' show
     Target;
 
 import 'ticker.dart' show
     Ticker;
 
 import 'translate_uri.dart' show
     TranslateUri;
 
 /// Provides the implementation details used by a loader for a target.
 abstract class TargetImplementation extends Target {
   final TranslateUri uriTranslator;
+  Builder cachedCompileTimeError;
 
   TargetImplementation(Ticker ticker, this.uriTranslator)
       : super(ticker);
 
   /// Creates a [LibraryBuilder] corresponding to [uri], if one doesn't exist
   /// already.
   LibraryBuilder createLibraryBuilder(Uri uri, Uri fileUri);
 
   /// Add the classes extended or implemented directly by [cls] to [set].
   void addDirectSupertype(ClassBuilder cls, Set<ClassBuilder> set);
 
   /// Returns all classes that will be included in the resulting program.
   List<ClassBuilder> collectAllClasses();
 
   /// The class [cls] is involved in a cyclic definition. This method should
   /// ensure that the cycle is broken, for example, by removing superclass and
   /// implemented interfaces.
   void breakCycle(ClassBuilder cls);
 
   Uri translateUri(Uri uri) => uriTranslator.translate(uri);
+
+  /// Returns a reference to the constructor used for creating a compile-time
+  /// error. The constructor is expected to accept a single argument of type
+  /// String, which is the compile-time error message.
+  Builder getCompileTimeError(Loader loader) {
+    if (cachedCompileTimeError != null) return cachedCompileTimeError;
+    return cachedCompileTimeError =
+        loader.coreLibrary.getConstructor("_CompileTimeError", isPrivate: true);
+  }
 }