Parameterize TypeSystem by its node kind

pkg/compiler/lib/src/types/map_type_mask.dart

Index: pkg/compiler/lib/src/types/map_type_mask.dart
diff --git a/pkg/compiler/lib/src/types/map_type_mask.dart b/pkg/compiler/lib/src/types/map_type_mask.dart
index 5152b9d217b3fa942405d549f6702fd9d3f67e07..30e490a71ac030d661fd2dd0358bb6bab0dbca70 100644
--- a/pkg/compiler/lib/src/types/map_type_mask.dart
+++ b/pkg/compiler/lib/src/types/map_type_mask.dart
@@ -9,11 +9,11 @@ part of masks;
  * site of a map (currently only internal Map class) that will get specialized
  * once the [TypeGraphInferrer] phase finds a key and/or value type for it.
  */
-class MapTypeMask extends ForwardingTypeMask {
+class MapTypeMask<T> extends ForwardingTypeMask {
   final TypeMask forwardTo;
 
   // The [Node] where this type mask was created.
-  final Node allocationNode;
+  final T allocationNode;
 
   // The [MemberEntity] where this type mask was created.
   final MemberEntity allocationElement;