Use DiagnosticMessage in MockCompiler.

tests/compiler/dart2js/duplicate_library_test.dart

Index: tests/compiler/dart2js/duplicate_library_test.dart
diff --git a/tests/compiler/dart2js/duplicate_library_test.dart b/tests/compiler/dart2js/duplicate_library_test.dart
index 80aa6fecd0821528975489fc87817c952298aa5b..5a2a708f4e77b20432c1e1cbb767edd9cb629a08 100644
--- a/tests/compiler/dart2js/duplicate_library_test.dart
+++ b/tests/compiler/dart2js/duplicate_library_test.dart
@@ -13,13 +13,13 @@ import 'package:compiler/src/diagnostics/messages.dart' show MessageKind;
 import 'memory_compiler.dart';
 
 void check(String kind,
-           Iterable<DiagnosticMessage> messages,
+           Iterable<CollectedMessage> messages,
            List<MessageKind> expectedMessageKinds) {
   Expect.equals(messages.length, expectedMessageKinds.length,
       "Unexpected $kind count: $messages");
   int i = 0;
-  messages.forEach((DiagnosticMessage message) {
-    Expect.equals(expectedMessageKinds[i++], message.message.kind);
+  messages.forEach((CollectedMessage message) {
+    Expect.equals(expectedMessageKinds[i++], message.messageKind);
   });
 }