Use entities in all masks.

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

Index: pkg/compiler/lib/src/types/container_type_mask.dart
diff --git a/pkg/compiler/lib/src/types/container_type_mask.dart b/pkg/compiler/lib/src/types/container_type_mask.dart
index 908996bc2d392a6dd23274dc2b01b6a403856324..edad3af22c807312b5261506ad192b87199b529b 100644
--- a/pkg/compiler/lib/src/types/container_type_mask.dart
+++ b/pkg/compiler/lib/src/types/container_type_mask.dart
@@ -13,8 +13,8 @@ class ContainerTypeMask extends ForwardingTypeMask {
   // The [Node] where this type mask was created.
   final Node allocationNode;
 
-  // The [Element] where this type mask was created.
-  final Element allocationElement;
+  // The [Entity] where this type mask was created.
+  final Entity allocationElement;
 
   // The element type of this container.
   final TypeMask elementType;