Add prefix constraints for deferred imports.

tests/compiler/dart2js/deferred_emit_type_checks_test.dart

Index: tests/compiler/dart2js/deferred_emit_type_checks_test.dart
diff --git a/tests/compiler/dart2js/deferred_emit_type_checks_test.dart b/tests/compiler/dart2js/deferred_emit_type_checks_test.dart
index 890397ab8f23d29c823ea561c3953c11b0e47143..45955b56767061621a70b35e7058b70fb5c7f092 100644
--- a/tests/compiler/dart2js/deferred_emit_type_checks_test.dart
+++ b/tests/compiler/dart2js/deferred_emit_type_checks_test.dart
@@ -65,13 +65,13 @@ void main() {
 const Map MEMORY_SOURCE_FILES = const {"main.dart": """
 import "dart:async";
 
-@def import 'lib.dart' show f, A;
+@def import 'lib.dart' as lib show f, A;
 
 const def = const DeferredLibrary("deferred");
 
 void main() {
   def.load().then((_) {
-    print(f(new A<A>()));
+    print(lib.f(new lib.A<lib.A>()));
   });
 }
 """, "lib.dart": """