content [nfc]: Use Dart patterns for parsing math content

Author: rajveermalviya

lib/model/content.dart

@@ -873,41 +873,48 @@ String? _parseMath(dom.Element element, {required bool block}) {
   } else {
     assert(element.localName == 'span' && element.className == 'katex-display');
 
-    if (element.nodes.length != 1) return null;
-    final child = element.nodes.single;
-    if (child is! dom.Element) return null;
-    if (child.localName != 'span') return null;
-    if (child.className != 'katex') return null;
-    katexElement = child;
+    if (element.nodes case [
+      dom.Element(localName: 'span', className: 'katex') && final child,
+    ]) {
+      katexElement = child;
+    } else {
+      return null;
+    }
   }
 
   // Expect two children span.katex-mathml, span.katex-html .
   // For now we only care about the .katex-mathml .
-  if (katexElement.nodes.isEmpty) return null;
-  final child = katexElement.nodes.first;
-  if (child is! dom.Element) return null;
-  if (child.localName != 'span') return null;
-  if (child.className != 'katex-mathml') return null;
-
-  if (child.nodes.length != 1) return null;
-  final grandchild = child.nodes.single;
-  if (grandchild is! dom.Element) return null;
-  if (grandchild.localName != 'math') return null;
-  if (grandchild.attributes['display'] != (block ? 'block' : null)) return null;
-  if (grandchild.namespaceUri != 'http://www.w3.org/1998/Math/MathML') return null;
-
-  if (grandchild.nodes.length != 1) return null;
-  final greatgrand = grandchild.nodes.single;
-  if (greatgrand is! dom.Element) return null;
-  if (greatgrand.localName != 'semantics') return null;
-
-  if (greatgrand.nodes.isEmpty) return null;
-  final descendant4 = greatgrand.nodes.last;
-  if (descendant4 is! dom.Element) return null;
-  if (descendant4.localName != 'annotation') return null;
-  if (descendant4.attributes['encoding'] != 'application/x-tex') return null;
-
-  return descendant4.text.trim();
+  if (katexElement.nodes case [
+    dom.Element(localName: 'span', className: 'katex-mathml', nodes: [
+      dom.Element(
+        localName: 'math',
+        namespaceUri: 'http://www.w3.org/1998/Math/MathML')
+          && final mathElement,
+    ]),
+    ...
+  ]) {
+    if (mathElement.attributes['display'] != (block ? 'block' : null)) {
+      return null;
+    }
+
+    final String texSource;
+    if (mathElement.nodes case [
+      dom.Element(localName: 'semantics', nodes: [
+        ...,
+        dom.Element(
+          localName: 'annotation',
+          attributes: {'encoding': 'application/x-tex'},
+          :final text),
+      ]),
+    ]) {
+      texSource = text.trim();
+    } else {
+      return null;
+    }
+    return texSource;
+  } else {
+    return null;
+  }
 }
 
 /// Parser for the inline-content subtrees within Zulip content HTML.