Add dummy dart:html library for documentation.

lib/html/doc/interface/SVGAnimationElement.dartdoc

Index: lib/html/doc/interface/SVGAnimationElement.dartdoc
diff --git a/lib/html/doc/interface/SVGAnimationElement.dartdoc b/lib/html/doc/interface/SVGAnimationElement.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 SVGAnimationElement
+interface SVGAnimationElement extends SVGElement, SVGTests, SVGExternalResourcesRequired, ElementTimeControl {
+
+  /** @domName SVGAnimationElement.targetElement */
+  final SVGElement targetElement;
+
+  /** @domName SVGAnimationElement.getCurrentTime */
+  num getCurrentTime();
+
+  /** @domName SVGAnimationElement.getSimpleDuration */
+  num getSimpleDuration();
+
+  /** @domName SVGAnimationElement.getStartTime */
+  num getStartTime();
+}