Support directories other than "web" in pub build.

sdk/lib/_internal/pub/test/build/allows_arbitrary_modes_test.dart

Index: sdk/lib/_internal/pub/test/build/allows_arbitrary_modes_test.dart
diff --git a/sdk/lib/_internal/pub/test/build/allows_arbitrary_modes_test.dart b/sdk/lib/_internal/pub/test/build/allows_arbitrary_modes_test.dart
index 8a5f6bf7ac7811547fe66b86912fca1eeb9ba55d..a89b8699d80c3014c9dd0fefe5df5b8b858816b4 100644
--- a/sdk/lib/_internal/pub/test/build/allows_arbitrary_modes_test.dart
+++ b/sdk/lib/_internal/pub/test/build/allows_arbitrary_modes_test.dart
@@ -51,7 +51,9 @@ main() {
 
     d.dir(appPath, [
       d.dir('build', [
-        d.file('foo.out', 'depeche')
+        d.dir('web', [
+          d.file('foo.out', 'depeche')
+        ])
       ])
     ]).validate();
   });