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

tests/compiler/dart2js/mock_compiler.dart

Index: tests/compiler/dart2js/mock_compiler.dart
diff --git a/tests/compiler/dart2js/mock_compiler.dart b/tests/compiler/dart2js/mock_compiler.dart
index c094d864cbb2ad1f44274888f0e33ba515fcaf3a..b4494bc907b372596af9ad8e6ee865e7d91e2a41 100644
--- a/tests/compiler/dart2js/mock_compiler.dart
+++ b/tests/compiler/dart2js/mock_compiler.dart
@@ -470,3 +470,19 @@ class MockDeferredLoadTask extends DeferredLoadTask {
     // 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);
+    }
+  };
+}