add fromElement constructor

Author: andy1xx8

lib/flutter_html.dart

@@ -73,7 +73,29 @@ class Html extends StatelessWidget {
   Html.fromDom({
     Key? key,
     GlobalKey? anchorKey,
-    @required this.document,
+    @required dom.Document? document,
+    this.onLinkTap,
+    this.onAnchorTap,
+    this.customRender = const {},
+    this.customImageRenders = const {},
+    this.onCssParseError,
+    this.onImageError,
+    this.onMathError,
+    this.shrinkWrap = false,
+    this.onImageTap,
+    this.tagsList = const [],
+    this.style = const {},
+    this.navigationDelegateForIframe,
+  })  : data = null,
+        assert(document != null),
+        this.document = document!.documentElement,
+        _anchorKey = anchorKey ?? GlobalKey(),
+        super(key: key);
+
+  Html.fromElement({
+    Key? key,
+    GlobalKey? anchorKey,
+    this.document,
     this.onLinkTap,
     this.onAnchorTap,
     this.customRender = const {},