Leave only widget creation tracker kernel pass in flutter frontend se…

…rver. (flutter#4727)

* Leave only widget creation tracker kernel pass in flutter frontend server.

This is follow-up to https://dart-review.googlesource.com/c/sdk/+/43888.

* Adapt to updated interface

* Tyop

* Fix imports. Compiler interface extension

* Add trivial test back. Rename library to flutter_frontend_server.

flutter_kernel_transformers/lib/track_widget_constructor_locations.dart

@@ -12,6 +12,7 @@ library track_widget_constructor_locations;
 import 'package:kernel/ast.dart';
 import 'package:kernel/class_hierarchy.dart';
 import 'package:meta/meta.dart';
+import 'package:vm/frontend_server.dart' show ProgramTransformer;
 
 // Parameter name used to track were widget constructor calls were made from.
 //
@@ -285,7 +286,7 @@ class _WidgetCallSiteTransformer extends Transformer {
 /// The creation location is stored as a private field named `_location`
 /// on the base widget class and flowed through the constructors using a named
 /// parameter.
-class WidgetCreatorTracker {
+class WidgetCreatorTracker implements ProgramTransformer {
   Class _widgetClass;
   Class _locationClass;
 
@@ -440,6 +441,7 @@ class WidgetCreatorTracker {
   ///
   /// It is safe to call this method on a delta program generated as part of
   /// performing a hot reload.
+  @override
   void transform(Program program) {
     final List<Library> libraries = program.libraries;