Add prefix constraints for deferred imports.

tests/language/deferred_constraints_lib2.dart

Index: tests/language/deferred_constraints_lib2.dart
diff --git a/tests/compiler/dart2js/path with spaces/library space/part space/part space.dart b/tests/language/deferred_constraints_lib2.dart
similarity index 87%
copy from tests/compiler/dart2js/path with spaces/library space/part space/part space.dart
copy to tests/language/deferred_constraints_lib2.dart
index 58aff0fb6e8a8e53197e400cf859718ce5e65606..73521264727218aa0363d2671721e7532fd4412f 100644
--- a/tests/compiler/dart2js/path with spaces/library space/part space/part space.dart	
+++ b/tests/language/deferred_constraints_lib2.dart
@@ -2,6 +2,4 @@
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
-part of spaces;
-
-bar() => 499;
+foo () => 42;