Add dummy dart:html library for documentation.

lib/html/doc/interface/SVGGradientElement.dartdoc

+// 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:
+//   This file contains documentation that is merged into the real source.
+//   Do not make code changes here.
+
+/// @domName SVGGradientElement
+interface SVGGradientElement extends SVGElement, SVGURIReference, SVGExternalResourcesRequired, SVGStylable {
+
+  static final int SVG_SPREADMETHOD_PAD = 1;
+
+  static final int SVG_SPREADMETHOD_REFLECT = 2;
+
+  static final int SVG_SPREADMETHOD_REPEAT = 3;
+
+  static final int SVG_SPREADMETHOD_UNKNOWN = 0;
+
+  /** @domName SVGGradientElement.gradientTransform */
+  final SVGAnimatedTransformList gradientTransform;
+
+  /** @domName SVGGradientElement.gradientUnits */
+  final SVGAnimatedEnumeration gradientUnits;
+
+  /** @domName SVGGradientElement.spreadMethod */
+  final SVGAnimatedEnumeration spreadMethod;
+}