Support use of dill in TypeEnv tests

tests/compiler/dart2js/union_type_test.dart

Index: tests/compiler/dart2js/union_type_test.dart
diff --git a/tests/compiler/dart2js/union_type_test.dart b/tests/compiler/dart2js/union_type_test.dart
index 1c9ec4afed63550a458ae40b0e393a881e063a58..8f6259a0dc87fe1737eb7935103475d48cc3c972 100644
--- a/tests/compiler/dart2js/union_type_test.dart
+++ b/tests/compiler/dart2js/union_type_test.dart
@@ -18,7 +18,7 @@ main() {
         new A();
         new B();
       }
-      """, useMockCompiler: false);
+      """, compileMode: CompileMode.memory);
     ClosedWorld world = env.closedWorld;
     FlatTypeMask mask1 = new FlatTypeMask.exact(env.getClass('A'));
     FlatTypeMask mask2 = new FlatTypeMask.exact(env.getClass('B'));