Support use of dill in TypeEnv tests

tests/compiler/dart2js/class_set_test.dart

Index: tests/compiler/dart2js/class_set_test.dart
diff --git a/tests/compiler/dart2js/class_set_test.dart b/tests/compiler/dart2js/class_set_test.dart
index 54caa73a1f81cba783ad27dc5d83b4ea0b682811..02b439f81e6a6f2818dc9eaaa5b0fec1672f731e 100644
--- a/tests/compiler/dart2js/class_set_test.dart
+++ b/tests/compiler/dart2js/class_set_test.dart
@@ -46,7 +46,7 @@ testIterators() async {
         new F();
         new G();
       }
-      """, useMockCompiler: false);
+      """, compileMode: CompileMode.memory);
   ClosedWorld world = env.closedWorld;
 
   ClassElement A = env.getElement("A");
@@ -376,7 +376,7 @@ testForEach() async {
         new H();
         new I();
       }
-      """, useMockCompiler: false);
+      """, compileMode: CompileMode.memory);
   ClosedWorld world = env.closedWorld;
 
   ClassElement A = env.getElement("A");