Fix summary handling of unresolved imports, exports, and parts.

pkg/analyzer/test/src/summary/resynthesize_strong_test.dart

 // Copyright (c) 2016, the Dart project authors.  Please see the AUTHORS file
 // 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 analyzer.test.src.summary.resynthesize_strong_test;
 
 import 'package:analyzer/src/generated/engine.dart';
 import 'package:analyzer/src/generated/sdk.dart';
 import 'package:unittest/unittest.dart';
 
 import '../../reflective_tests.dart';
 import '../context/abstract_context.dart';
 import 'resynthesize_test.dart';
 
 main() {
   groupSep = ' | ';
   runReflectiveTests(ResynthesizeStrongTest);
 }
 
 @reflectiveTest
 class ResynthesizeStrongTest extends ResynthesizeElementTest {
   @override
+  bool allowMissingFiles = false;
+
+  @override
   DartSdk createDartSdk() => AbstractContextTest.SHARED_STRONG_MOCK_SDK;
 
   @override
   AnalysisOptionsImpl createOptions() =>
       super.createOptions()..strongMode = true;
 }