Add dummy dart:html library for documentation.

lib/html/doc/interface/ParamElement.dartdoc

Index: lib/html/doc/interface/ParamElement.dartdoc
diff --git a/lib/html/doc/interface/ParamElement.dartdoc b/lib/html/doc/interface/ParamElement.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 HTMLParamElement
+interface ParamElement extends Element default _Elements {
+
+  ParamElement();
+
+  /** @domName HTMLParamElement.name */
+  String name;
+
+  /** @domName HTMLParamElement.type */
+  String type;
+
+  /** @domName HTMLParamElement.value */
+  String value;
+
+  /** @domName HTMLParamElement.valueType */
+  String valueType;
+}