Add dummy dart:html library for documentation.

lib/html/doc/interface/Rect.dart

Index: lib/html/doc/interface/Rect.dart
diff --git a/lib/html/doc/interface/Rect.dart b/lib/html/doc/interface/Rect.dart
new file mode 100644
index 0000000000000000000000000000000000000000..39df2a5c822d8b55329c1e035dbc6d447c5ff835
--- /dev/null
+++ b/lib/html/doc/interface/Rect.dart
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+// 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 Rect
+interface Rect {
+
+  /** @domName Rect.bottom */
+  final CSSPrimitiveValue bottom;
+
+  /** @domName Rect.left */
+  final CSSPrimitiveValue left;
+
+  /** @domName Rect.right */
+  final CSSPrimitiveValue right;
+
+  /** @domName Rect.top */
+  final CSSPrimitiveValue top;
+}