content [nfc]: Move static _parseMath out from parser class

Because this is used for parsing both inline and block nodes,
moving it out will help us separate the rest of the parser code
between the handling of inline content and block content.

Author: gnprice

lib/model/content.dart

@@ -806,6 +806,52 @@ class GlobalTimeNode extends InlineContentNode {
 
 ////////////////////////////////////////////////////////////////
 
+String? _parseMath(dom.Element element, {required bool block}) {
+  final dom.Element katexElement;
+  if (!block) {
+    assert(element.localName == 'span' && element.className == 'katex');
+
+    katexElement = element;
+  } 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;
+  }
+
+  // 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();
+}
+
 /// What sort of nodes a [_ZulipContentParser] is currently expecting to find.
 enum _ParserContext {
   /// The parser is currently looking for block nodes.
@@ -825,52 +871,6 @@ class _ZulipContentParser {
   /// and should be read or updated only inside an assertion.
   _ParserContext _debugParserContext = _ParserContext.block;
 
-  static String? _parseMath(dom.Element element, {required bool block}) {
-    final dom.Element katexElement;
-    if (!block) {
-      assert(element.localName == 'span' && element.className == 'katex');
-
-      katexElement = element;
-    } 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;
-    }
-
-    // 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();
-  }
-
   String? parseInlineMath(dom.Element element) {
     assert(_debugParserContext == _ParserContext.inline);
     return _parseMath(element, block: false);