[dart:html] Add MathMLElement

Fixes b/236721367

Per https://developer.mozilla.org/en-US/docs/Web/API/MathMLElement,
MathMLElement is in the Element type hierarchy. This adds an empty
class so that type-checking against Element can pass.

Change-Id: Ie0c2e33c2236bedd5627ed8624437d24d9fbac2d
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/249945
Commit-Queue: Srujan Gaddam <srujzs@google.com>
Reviewed-by: Sigmund Cherem <sigmund@google.com>

sdk/lib/html/dart2js/html_dart2js.dart

@@ -20555,6 +20555,23 @@ class MapElement extends HtmlElement {
 // 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.
 
+@Native("MathMLElement")
+class MathMLElement extends Element {
+  // To suppress missing implicit constructor warnings.
+  factory MathMLElement._() {
+    throw new UnsupportedError("Not supported");
+  }
+  /**
+   * Constructor instantiated by the DOM when a custom element has been created.
+   *
+   * This can only be called by subclasses from their created constructor.
+   */
+  MathMLElement.created() : super.created();
+}
+// 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.
+
 @Native("MediaCapabilities")
 class MediaCapabilities extends JavaScriptObject {
   // To suppress missing implicit constructor warnings.

tools/dom/dom.json

@@ -12423,6 +12423,10 @@
     },
     "support_level": "untriaged"
   },
+  "MathMLElement": {
+    "members": {},
+    "support_level": "untriaged"
+  },
   "Matrix": {
     "members": {
       "Matrix": {},

tools/dom/idl/dart/dart.idl

@@ -681,3 +681,5 @@ interface NoncedElement {
 // PositionCallback can be used on a deprecated Position object,
 // a GeolocationPosition object, or a Firefox-specific object.
 callback PositionCallback = void(object position);
+
+interface MathMLElement : Element {};