Add "not intended to be implemented/extended" warnings to pkg/front_end.

pkg/front_end/lib/compilation_error.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.
 
 /// Defines the API for the front end to communicate information about
 /// compilation errors to clients.
 library front_end.compilation_error;
 
 import 'package:source_span/source_span.dart' show SourceSpan;
 
 /// A single error that occurred during compilation, and information about where
 /// it occurred.
 ///
 /// TODO(paulberry): add a reference to the analyzer error code.
 ///
 /// TODO(paulberry): add a correction message, once most analyzer errors have
 /// one.
+///
+/// Not intended to be implemented or extended by clients.
 abstract class CompilationError {
   /// A text description of the compile error.
   String get message;
 
   /// The source location where the error occurred.
   SourceSpan get location;
 }