Handle invalid deferred prefix declarations.

tests/compiler/dart2js/serialization/test_data.dart

Index: tests/compiler/dart2js/serialization/test_data.dart
diff --git a/tests/compiler/dart2js/serialization/test_data.dart b/tests/compiler/dart2js/serialization/test_data.dart
index e0fca607d07630bbfa0b1212d5c21d7617ab6750..74ec44f5c161a0f2d9b43ccc4aa1280b0d89dbdb 100644
--- a/tests/compiler/dart2js/serialization/test_data.dart
+++ b/tests/compiler/dart2js/serialization/test_data.dart
@@ -484,7 +484,7 @@ class D = A with B, C;
 
   const Test('Deferred prefix loadLibrary', const {
     'main.dart': '''
-    import 'a.dart';
+import 'a.dart';
 
 main() {
   test();
@@ -500,6 +500,43 @@ test() {
     'b.dart': '''
 ''',
   }),
+
+  const Test('Deferred without prefix', const {
+    'main.dart': '''
+import 'a.dart';
+
+main() {
+  test();
+}
+''',
+  }, preserializedSourceFiles: const {
+    'a.dart': '''
+import 'b.dart' deferred;
+test() {}
+''',
+    'b.dart': '''
+''',
+  }, expectedErrorCount: 1),
+
+  const Test('Deferred with duplicate prefix', const {
+    'main.dart': '''
+import 'a.dart';
+
+main() {
+  test();
+}
+''',
+  }, preserializedSourceFiles: const {
+    'a.dart': '''
+import 'b.dart' deferred as pre;
+import 'c.dart' deferred as pre;
+test() {}
+''',
+    'b.dart': '''
+''',
+    'c.dart': '''
+''',
+  }, expectedErrorCount: 1),
 ];
 
 class Test {