Add prefix constraints for deferred imports.

tests/language/duplicate_import_prefix_test.dart

Index: tests/language/duplicate_import_prefix_test.dart
diff --git a/tests/compiler/dart2js/path with spaces/library space/lib with spaces.dart b/tests/language/duplicate_import_prefix_test.dart
similarity index 59%
copy from tests/compiler/dart2js/path with spaces/library space/lib with spaces.dart
copy to tests/language/duplicate_import_prefix_test.dart
index 16da6e3295bca2daa89abcf7104d7a492f7cd846..9c288029ab40a7aee66ffeb1f67467e34bdc80fc 100644
--- a/tests/compiler/dart2js/path with spaces/library space/lib with spaces.dart	
+++ b/tests/language/duplicate_import_prefix_test.dart
@@ -2,8 +2,9 @@
 // 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.
 
-library spaces;
+// Importing with a duplicate prefix is allowed.
 
-part "part space/part space.dart";
+import "duplicate_import_liba.dart" as a;
+import "duplicate_import_libb.dart" as a;
 
-foo() => bar();
+void main() {}