Add prefix constraints for deferred imports.

tests/language/deferred_constraints_lib.dart

Index: tests/language/deferred_constraints_lib.dart
diff --git a/tests/compiler/dart2js/path with spaces/library space/lib with spaces.dart b/tests/language/deferred_constraints_lib.dart
similarity index 72%
copy from tests/compiler/dart2js/path with spaces/library space/lib with spaces.dart
copy to tests/language/deferred_constraints_lib.dart
index 16da6e3295bca2daa89abcf7104d7a492f7cd846..cdc9d4d8fbddf56553e4e04f5d44eeac2534fa3a 100644
--- a/tests/compiler/dart2js/path with spaces/library space/lib with spaces.dart	
+++ b/tests/language/deferred_constraints_lib.dart
@@ -2,8 +2,10 @@
 // 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;
+class C {}
 
-part "part space/part space.dart";
+class Const {
+  const Const();
+}
 
-foo() => bar();
+const constantInstance = const Const();