Add prefix constraints for deferred imports.

tests/language/deferred_duplicate_prefix2_negative_test.dart

Index: tests/language/deferred_duplicate_prefix2_negative_test.dart
diff --git a/tests/lib/mirrors/method_mirror_source_line_ending_lf.dart b/tests/language/deferred_duplicate_prefix2_negative_test.dart
old mode 100755
new mode 100644
similarity index 54%
copy from tests/lib/mirrors/method_mirror_source_line_ending_lf.dart
copy to tests/language/deferred_duplicate_prefix2_negative_test.dart
index 71a34c82d29a17993417f8641a1bc38e63e5f11f..1cb9dace70d72e9e967d66373ac58879f68fb5af
--- a/tests/lib/mirrors/method_mirror_source_line_ending_lf.dart
+++ b/tests/language/deferred_duplicate_prefix2_negative_test.dart
@@ -2,15 +2,11 @@
 // 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.
 
-// This file is marked executable so source control will preserve the type of
-// line ending.
+import "dart:async";
 
-library line_endings.lf;
+@lazy import "deferred_constraints_lib.dart" as lib;
+import "deferred_constraints_lib2.dart" as lib;
 
-oneLineLF(x) => x;
-multiLineLF(y) {
-  return y + 1;
-}
-a
-(){
-}
+const lazy = const DeferredLibrary('lib');
+
+void main() {}