Handle missing imports/exports/parts.

tests/compiler/dart2js/message_kind_helper.dart

Index: tests/compiler/dart2js/message_kind_helper.dart
diff --git a/tests/compiler/dart2js/message_kind_helper.dart b/tests/compiler/dart2js/message_kind_helper.dart
index f972a96fc57cf96bb752a8e8e42f6e49c1c3c1ee..6302bf190608302f6e25b8b008093c02377840c4 100644
--- a/tests/compiler/dart2js/message_kind_helper.dart
+++ b/tests/compiler/dart2js/message_kind_helper.dart
@@ -14,8 +14,13 @@ 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) {
@@ -37,7 +42,9 @@ Compiler check(MessageKind kind, Compiler cachedCompiler) {
 
     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'#\\\{[^}]*\\\}'), '.*');