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

pkg/front_end/lib/resolved_ast_generator.dart

Index: pkg/front_end/lib/resolved_ast_generator.dart
diff --git a/pkg/front_end/lib/resolved_ast_generator.dart b/pkg/front_end/lib/resolved_ast_generator.dart
index 3492a70da539ee83c7747e8d1bc7b9f734415f5f..4b37d3888af61c2fd56a60c1d681e0440b8b0ce4 100644
--- a/pkg/front_end/lib/resolved_ast_generator.dart
+++ b/pkg/front_end/lib/resolved_ast_generator.dart
@@ -38,6 +38,8 @@ Future<ResolvedAsts> resolvedAstsFor(
     throw new UnimplementedError();
 
 /// Representation of the resolved ASTs of a build unit.
+///
+/// Not intended to be implemented or extended by clients.
 @deprecated
 abstract class ResolvedAsts {
   /// The resolved ASTs of the build unit's source libraries.