Detect concurrent edits.

dart/site/try/src/ui.dart

Index: dart/site/try/src/ui.dart
diff --git a/dart/site/try/src/ui.dart b/dart/site/try/src/ui.dart
index d8a93a648bd6c70589efdd67e5794df4134b8fed..e9b66d654cc20358a3d4f3c20609407de9114d8a 100644
--- a/dart/site/try/src/ui.dart
+++ b/dart/site/try/src/ui.dart
@@ -48,6 +48,7 @@ import 'compilation.dart' show
 InteractionManager interaction;
 
 DivElement mainEditorPane;
+DivElement statusDiv;
 PreElement outputDiv;
 DivElement hackDiv;
 IFrameElement outputFrame;
@@ -130,6 +131,13 @@ buildUI() {
       ..position = 'absolute';
   inputWrapper.append(inputHeader);
 
+  statusDiv = new DivElement();
+  statusDiv.style
+      ..left = '0px'
+      ..top = '0px'
+      ..position = 'absolute';
+  inputWrapper.append(statusDiv);
+
   outputFrame =
       makeOutputFrame(
           Url.createObjectUrl(new Blob([''], 'application/javascript')));