Handle missing imports/exports/parts.

tests/compiler/dart2js/message_kind_helper.dart

 // Copyright (c) 2013, 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 dart2js.test.message_kind_helper;
 
 import 'package:expect/expect.dart';
 
 import '../../../sdk/lib/_internal/compiler/implementation/dart2jslib.dart' show
     Compiler,
     MessageKind;
 
 import 'memory_compiler.dart';
 
 const String ESCAPE_REGEXP = r'[[\]{}()*+?.\\^$|]';
 
-Compiler check(MessageKind kind, Compiler cachedCompiler) {
-  Expect.isNotNull(kind.howToFix);
+Compiler check(MessageKind kind, Compiler cachedCompiler,
+               {bool expectNoHowToFix: false}) {
+  if (expectNoHowToFix) {
+    Expect.isNull(kind.howToFix);
+  } else {
+    Expect.isNotNull(kind.howToFix);
+  }
   Expect.isFalse(kind.examples.isEmpty);
 
   for (String example in kind.examples) {
     List<String> messages = <String>[];
     void collect(Uri uri, int begin, int end, String message, kind) {
       if (kind.name == 'verbose info') {
         return;
       }
       messages.add(message);
     }
 
     Compiler compiler = compilerFor(
         {'main.dart': example},
         diagnosticHandler: collect,
         options: ['--analyze-only'],
         cachedCompiler: cachedCompiler);
 
     compiler.run(Uri.parse('memory:main.dart'));
 
     Expect.isFalse(messages.isEmpty, 'No messages in """$example"""');
 
-    String pattern = '${kind.template}\n${kind.howToFix}'.replaceAllMapped(
+    String expectedText = kind.howToFix == null
+        ? kind.template : '${kind.template}\n${kind.howToFix}';
+    String pattern = expectedText.replaceAllMapped(
         new RegExp(ESCAPE_REGEXP), (m) => '\\${m[0]}');
     pattern = pattern.replaceAll(new RegExp(r'#\\\{[^}]*\\\}'), '.*');
 
     for (String message in messages) {
       Expect.isTrue(new RegExp('^$pattern\$').hasMatch(message),
                     '"$pattern" does not match "$message"');
     }
     return compiler;
   }
 }