Add generated element constructors (#185)

For every tag that the interface supports, emits a Dart
constructor that can be used to create an element of that tag
using either createElement or createElementNS. Adds a dependency
to @webref/elements to fetch the element data.

Note that not every element interface has a tag associated with it.

Adds a test and modifies existing helper messages to
point to the new constructors instead.

Also deprecates createAudioElement.

CHANGELOG.md

@@ -4,6 +4,10 @@
 - Include MDN API documentation as class and member dartdoc comments. Docs
   sourced from the [MDN Web Docs][] project; attributions and copyright
   licensing by Mozilla Contributors is licensed under [CC-BY-SA 2.5][].
+- Add a constructor for each Element tag onto their respective Element
+  interfaces.
+- Remove `external` Element constructors that would result in a runtime error.
+- Deprecate `createAudioElement` in favor of the `HTMLAudioElement` constructor.
 
 [MDN Web Docs]: https://developer.mozilla.org/en-US/docs/Web
 [CC-BY-SA 2.5]: https://creativecommons.org/licenses/by-sa/2.5/

lib/src/dom/css_masking.dart

@@ -13,19 +13,34 @@ library;
 
 import 'dart:js_interop';
 
+import 'dom.dart';
 import 'svg.dart';
 
 /// The **`SVGClipPathElement`** interface provides access to the properties of
 /// elements, as well as methods to manipulate them.
 extension type SVGClipPathElement._(JSObject _)
     implements SVGElement, JSObject {
+  /// Creates an [SVGClipPathElement] using the tag 'clipPath'.
+  SVGClipPathElement()
+      : _ = document.createElementNS(
+          'http://www.w3.org/2000/svg',
+          'clipPath',
+        );
+
   external SVGAnimatedEnumeration get clipPathUnits;
   external SVGAnimatedTransformList get transform;
 }
 
 /// The **`SVGMaskElement`** interface provides access to the properties of
 /// elements, as well as methods to manipulate them.
 extension type SVGMaskElement._(JSObject _) implements SVGElement, JSObject {
+  /// Creates an [SVGMaskElement] using the tag 'mask'.
+  SVGMaskElement()
+      : _ = document.createElementNS(
+          'http://www.w3.org/2000/svg',
+          'mask',
+        );
+
   external SVGAnimatedEnumeration get maskUnits;
   external SVGAnimatedEnumeration get maskContentUnits;
   external SVGAnimatedLength get x;