Fix issues that broke editing on browsers without Shadow DOM support.

dart/site/try/bugs/single_line_delete.dart

+// Copyright (c) 2014, 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.
+
+var greeting = "Hello, World!";
+
+void main() {
+  // Put cursor at end of previous line. Hit backspace.
+  // 1. Ensure this triggers a single-line change.
+  // 2. Ensure the cursor position is correct.
+  // Then restore the file and place the cursor at the end of the file. Delete
+  // each character in the file by holding down backspace. Verify that there
+  // are no exceptions and that the entire buffer is deleted.
+  // Then restore the file and place the cursor before the semicolon on the
+  // first line. Hit delete and verify that a character is deleted.
+  print(greeting);
+}