Support use of dill in TypeEnv tests

tests/compiler/dart2js/type_mask2_test.dart

Index: tests/compiler/dart2js/type_mask2_test.dart
diff --git a/tests/compiler/dart2js/type_mask2_test.dart b/tests/compiler/dart2js/type_mask2_test.dart
index 524a4009e07bd09ced7baab119cb75ef384eab5f..5c6e15998b90f17decae06f0511c8753b8b3106b 100644
--- a/tests/compiler/dart2js/type_mask2_test.dart
+++ b/tests/compiler/dart2js/type_mask2_test.dart
@@ -100,7 +100,7 @@ Future testUnionTypeMaskFlatten() async {
         new D();
         new E();
       }
-      """, useMockCompiler: false);
+      """, compileMode: CompileMode.memory);
 
   ClosedWorld closedWorld = env.closedWorld;
 
@@ -210,7 +210,7 @@ Future testStringSubtypes() async {
         '' is String;
       }
       """,
-      useMockCompiler: false);
+      compileMode: CompileMode.memory);
   ClosedWorld closedWorld = env.closedWorld;
 
   ClassElement Object_ = env.getElement("Object");