Adds Extension node to kernel/ast, currently with serialization. This
is needed to support resolution of extension methods through dill
boundaries.
Adds (Source)ExtensionBuilder for build Extension nodes and
DeclarationBuilder as common superclass of ClassBuilder and
ExtensionBuilder.
Change-Id: Id945a40452f2729f9a4f390477dd6c76f9030eaa
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/112256
Commit-Queue: Johnni Winther <johnniwinther@google.com>
Reviewed-by: Dmitry Stefantsov <dmitryas@google.com>
Johnni Winther
271ad96813