Declare local variables for the 'href' field, to promote and avoid printing 'null' (#3788)

Author: srawlins

lib/src/markdown_processor.dart

@@ -314,15 +314,12 @@ class MarkdownDocument extends md.Document {
     var textContent = _htmlEscape.convert(referenceText);
     var linkedElement = result.commentReferable;
     if (linkedElement != null) {
-      if (linkedElement.href != null) {
+      if (linkedElement.href case var href?) {
         var anchor = md.Element.text('a', textContent);
         if (linkedElement is ModelElement && linkedElement.isDeprecated) {
           anchor.attributes['class'] = 'deprecated';
         }
-        var href = linkedElement.href;
-        if (href != null) {
-          anchor.attributes['href'] = href;
-        }
+        anchor.attributes['href'] = href;
         return anchor;
       } else {
         // Otherwise this would be `linkedElement.linkedName`, but link bodies

lib/src/model/category.dart

@@ -143,6 +143,11 @@ class Category
   String get linkedName {
     final unbrokenName = name.replaceAll(' ', ' ');
     if (isDocumented) {
+      final href = this.href;
+      if (href == null) {
+        throw StateError("Requesting the 'linkedName' of a non-canonical "
+            "category: '$name'");
+      }
       return '<a href="$href">$unbrokenName</a>';
     } else {
       return unbrokenName;

lib/src/model/model_element.dart

@@ -687,6 +687,7 @@ abstract class ModelElement
         element.kind == ElementKind.NEVER ||
         this is ModelFunction);
 
+    final href = this.href;
     if (href == null) {
       if (isPublicAndPackageDocumented) {
         warn(PackageWarning.noCanonicalFound);