Fix merge error for newBuilder() invocation.

Change-Id: If8c470d1d142c27807cb7e9324df369d795930df
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/236304
Commit-Queue: Konstantin Shcheglov <[email protected]>
Reviewed-by: Phil Quitslund <[email protected]>

Author: scheglov

pkg/analyzer_plugin/test/src/utilities/change_builder/change_builder_dart_test.dart

@@ -324,7 +324,7 @@ class C {
     var path = convertPath('/home/test/lib/test.dart');
     addSource(path, '');
 
-    var builder = newBuilder();
+    var builder = await newBuilder();
     await builder.addDartFileEdit(path, (builder) {
       builder.addInsertion(0, (builder) {
         builder.write('if (');