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

pkg/front_end/lib/compilation_error.dart

Index: pkg/front_end/lib/compilation_error.dart
diff --git a/pkg/front_end/lib/compilation_error.dart b/pkg/front_end/lib/compilation_error.dart
index 6233f0d2fd06152daf8d35e29ca200475d34bb6c..2211ae98c2237e5b9c5f76568047e71f9f362a0c 100644
--- a/pkg/front_end/lib/compilation_error.dart
+++ b/pkg/front_end/lib/compilation_error.dart
@@ -15,6 +15,8 @@ import 'package:source_span/source_span.dart' show SourceSpan;
 ///
 /// 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;