Check for non-final field in the face of const constructors.

tests/compiler/dart2js/mock_compiler.dart

 // Copyright (c) 2012, 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 mock_compiler;
 
 import "package:expect/expect.dart";
 import 'dart:async';
 import 'dart:collection';
 
@@ skipping 452 matching lines @@
   }
 }
 
 class MockDeferredLoadTask extends DeferredLoadTask {
   MockDeferredLoadTask(Compiler compiler) : super(compiler);
 
   void registerMainApp(LibraryElement mainApp) {
     // Do nothing.
   }
 }
+
+api.DiagnosticHandler createHandler(MockCompiler compiler, String text) {
+  return (uri, int begin, int end, String message, kind) {
+    SourceFile sourceFile;
+    if (uri == null) {
+      sourceFile = new SourceFile('analysis', text);
+    } else {
+      sourceFile = compiler.sourceFiles[uri.toString()];
+    }
+    if (sourceFile != null && begin != null && end != null) {
+      print(sourceFile.getLocationMessage(message, begin, end, true, (x) => x));
+    } else {
+      print(message);
+    }
+  };
+}