Add dummy dart:html library for documentation.

lib/html/doc/interface/SVGRadialGradientElement.dartdoc

Index: lib/html/doc/interface/SVGRadialGradientElement.dartdoc
diff --git a/lib/html/doc/interface/SVGRadialGradientElement.dartdoc b/lib/html/doc/interface/SVGRadialGradientElement.dartdoc
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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:
+//   This file contains documentation that is merged into the real source.
+//   Do not make code changes here.
+
+/// @domName SVGRadialGradientElement
+interface SVGRadialGradientElement extends SVGGradientElement {
+
+  /** @domName SVGRadialGradientElement.cx */
+  final SVGAnimatedLength cx;
+
+  /** @domName SVGRadialGradientElement.cy */
+  final SVGAnimatedLength cy;
+
+  /** @domName SVGRadialGradientElement.fx */
+  final SVGAnimatedLength fx;
+
+  /** @domName SVGRadialGradientElement.fy */
+  final SVGAnimatedLength fy;
+
+  /** @domName SVGRadialGradientElement.r */
+  final SVGAnimatedLength r;
+}