[dart2js] Use enumset API for typemask bits.

Change-Id: I8f513d92d05290dceb53edc5f99a6d84a11a5571
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/399121
Reviewed-by: Nate Biggs <[email protected]>
Commit-Queue: Mayank Patke <[email protected]>

Author: fishythefish

pkg/compiler/lib/src/inferrer/typemasks/container_type_mask.dart

@@ -57,15 +57,16 @@ class ContainerTypeMask extends AllocationTypeMask {
   }
 
   @override
-  ContainerTypeMask withFlags({bool? isNullable, bool? hasLateSentinel}) {
+  ContainerTypeMask withSpecialValues(
+      {bool? isNullable, bool? hasLateSentinel}) {
     isNullable ??= this.isNullable;
     hasLateSentinel ??= this.hasLateSentinel;
     if (isNullable == this.isNullable &&
         hasLateSentinel == this.hasLateSentinel) {
       return this;
     }
     return ContainerTypeMask(
-        forwardTo.withFlags(
+        forwardTo.withSpecialValues(
             isNullable: isNullable, hasLateSentinel: hasLateSentinel),
         allocationNode,
         allocationElement,

pkg/compiler/lib/src/inferrer/typemasks/dictionary_type_mask.dart

@@ -52,15 +52,16 @@ class DictionaryTypeMask extends MapTypeMask {
   }
 
   @override
-  DictionaryTypeMask withFlags({bool? isNullable, bool? hasLateSentinel}) {
+  DictionaryTypeMask withSpecialValues(
+      {bool? isNullable, bool? hasLateSentinel}) {
     isNullable ??= this.isNullable;
     hasLateSentinel ??= this.hasLateSentinel;
     if (isNullable == this.isNullable &&
         hasLateSentinel == this.hasLateSentinel) {
       return this;
     }
     return DictionaryTypeMask(
-        forwardTo.withFlags(
+        forwardTo.withSpecialValues(
             isNullable: isNullable, hasLateSentinel: hasLateSentinel),
         allocationNode,
         allocationElement,