Use DiagnosticMessage in MockCompiler.

tests/compiler/dart2js/override_inheritance_test.dart

Index: tests/compiler/dart2js/override_inheritance_test.dart
diff --git a/tests/compiler/dart2js/override_inheritance_test.dart b/tests/compiler/dart2js/override_inheritance_test.dart
index c71e6b214668fae4d5b998f9ee1b1b7715cbac08..628f6253aa675563d6493d20e2f89f6d41c041fc 100644
--- a/tests/compiler/dart2js/override_inheritance_test.dart
+++ b/tests/compiler/dart2js/override_inheritance_test.dart
@@ -34,16 +34,24 @@ Future check(String source, {errors, warnings, hints, infos}) {
 
     toList(o) => o == null ? [] : o is List ? o : [o];
 
-    compareMessageKinds(source, toList(errors), compiler.errors, 'error');
+    compareMessageKinds(
+        source, toList(errors),
+        compiler.diagnosticCollector.errors, 'error');
 
-    compareMessageKinds(source, toList(warnings), compiler.warnings, 'warning');
+    compareMessageKinds(
+        source, toList(warnings),
+        compiler.diagnosticCollector.warnings, 'warning');
 
     if (infos != null) {
-      compareMessageKinds(source, toList(infos), compiler.infos, 'info');
+      compareMessageKinds(
+          source, toList(infos),
+          compiler.diagnosticCollector.infos, 'info');
     }
 
     if (hints != null) {
-      compareMessageKinds(source, toList(hints), compiler.hints, 'hint');
+      compareMessageKinds(
+          source, toList(hints),
+          compiler.diagnosticCollector.hints, 'hint');
     }
   });
 }