Fix errors in dart2js/source_map_pub_build_validity_test

tests/compiler/dart2js/source_map_pub_build_validity_test.dart

Index: tests/compiler/dart2js/source_map_pub_build_validity_test.dart
diff --git a/tests/compiler/dart2js/source_map_pub_build_validity_test.dart b/tests/compiler/dart2js/source_map_pub_build_validity_test.dart
index 69cf5e14ed360fe1b0e6322349f0c386cd9c1910..71fd04c9433f2001c456b5d8e80d212737b63424 100644
--- a/tests/compiler/dart2js/source_map_pub_build_validity_test.dart
+++ b/tests/compiler/dart2js/source_map_pub_build_validity_test.dart
@@ -11,7 +11,7 @@ import 'source_map_validator_helper.dart';
 
 void main() {
   asyncTest(() async {
-    Directory tmpDir = createTempDir();
+    Directory tmpDir = await createTempDir();
     try {
       Directory sunflowerDir = new Directory.fromUri(
           Platform.script.resolve('../../../third_party/sunflower'));
@@ -42,5 +42,5 @@ void main() {
     } finally {
       tmpDir.deleteSync(recursive: true);
     }
-  }));
+  });
 }