Add dummy dart:html library for documentation.

lib/html/doc/interface/ImageData.dart

Index: lib/html/doc/interface/ImageData.dart
diff --git a/lib/html/doc/interface/ImageData.dart b/lib/html/doc/interface/ImageData.dart
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index 0000000000000000000000000000000000000000..8d10032b73a86f57a1f94f40c7f6474c22656df6
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+++ b/lib/html/doc/interface/ImageData.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 ImageData
+interface ImageData {
+
+  /** @domName ImageData.data */
+  final Uint8ClampedArray data;
+
+  /** @domName ImageData.height */
+  final int height;
+
+  /** @domName ImageData.width */
+  final int width;
+}