Fix problem that caused deletion of tokens with multiple diagnostics.

R=johnniwinther@google.com

Review URL: https://codereview.chromium.org//374363002

git-svn-id: https://dart.googlecode.com/svn/branches/bleeding_edge/dart@38081 260f80e4-7a28-3924-810f-c04153c831b5

site/try/src/editor.dart

@@ -164,7 +164,7 @@ addDiagnostic(String kind, String message, int begin, int end) {
         Element parent = node.parent;
         if (parent.classes.contains("diagnostic") &&
             !interaction.oldDiagnostics.contains(parent)) {
-          Element other = parent.lastChild;
+          Element other = parent.firstChild;
           other.remove();
           SpanElement wrapper = new SpanElement();
           wrapper.style