Add dummy dart:html library for documentation.

lib/html/doc/interface/PreElement.dart

Index: lib/html/doc/interface/PreElement.dart
diff --git a/lib/html/doc/interface/PreElement.dart b/lib/html/doc/interface/PreElement.dart
new file mode 100644
index 0000000000000000000000000000000000000000..135ebde789e97652e25ddc61895514a3a7588ce3
--- /dev/null
+++ b/lib/html/doc/interface/PreElement.dart
@@ -0,0 +1,17 @@
+// 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.
+
+// WARNING: Do not edit - generated code.
+
+/// @domName HTMLPreElement
+interface PreElement extends Element default _Elements {
+
+  PreElement();
+
+  /** @domName HTMLPreElement.width */
+  int width;
+
+  /** @domName HTMLPreElement.wrap */
+  bool wrap;
+}