Add a link to the null safety documentation (#2455)

Author: parlough

lib/resources/styles.css

@@ -478,6 +478,10 @@ h1 .category {
   margin: 0 8px 0 0;
 }
 
+a.feature:hover {
+  border-color: #13B9FD;
+}
+
 h1 .feature {
   vertical-align: middle;
 }

lib/src/model/language_feature.dart

@@ -8,10 +8,15 @@ const Map<String, String> _featureDescriptions = {
   'Null safety': 'Supports the null safety language feature.',
 };
 
+const Map<String, String> _featureUrls = {
+  'Null safety': 'https://dart.dev/null-safety',
+};
+
 /// An abstraction for a language feature; used to render tags to notify
 /// the user that the documentation should be specially interpreted.
 class LanguageFeature {
   String get featureDescription => _featureDescriptions[name];
+  String get featureUrl => _featureUrls[name];
   String get featureLabel => _featureRenderer.renderFeatureLabel(this);
 
   final String name;

lib/src/render/feature_renderer.dart

@@ -10,6 +10,7 @@ abstract class FeatureRenderer {
 
 class FeatureRendererHtml extends FeatureRenderer {
   static final FeatureRendererHtml _instance = FeatureRendererHtml._();
+
   factory FeatureRendererHtml() {
     return _instance;
   }
@@ -18,27 +19,35 @@ class FeatureRendererHtml extends FeatureRenderer {
 
   @override
   String renderFeatureLabel(LanguageFeature feature) {
-    var spanClasses = <String>[];
-    spanClasses.add('feature');
-    spanClasses
-        .add('feature-${feature.name.split(' ').join('-').toLowerCase()}');
-
-    var buf = StringBuffer();
-    buf.write(
-        '<span class="${spanClasses.join(' ')}" title="${feature.featureDescription}">${feature.name}</span>');
-    return buf.toString();
+    final classesText = [
+      'feature',
+      'feature-${feature.name.split(' ').join('-').toLowerCase()}'
+    ].join(' ');
+
+    if (feature.featureUrl != null) {
+      return '<a href="${feature.featureUrl}" class="${classesText}"'
+          ' title="${feature.featureDescription}">${feature.name}</a>';
+    }
+
+    return '<span class="${classesText}" '
+        'title="${feature.featureDescription}">${feature.name}</span>';
   }
 }
 
 class FeatureRendererMd extends FeatureRenderer {
   static final FeatureRendererMd _instance = FeatureRendererMd._();
+
   factory FeatureRendererMd() {
     return _instance;
   }
 
   FeatureRendererMd._();
+
   @override
   String renderFeatureLabel(LanguageFeature feature) {
+    if (feature.featureUrl != null) {
+      return '*[\<${feature.name}\>](${feature.featureUrl})*';
+    }
     return '*\<${feature.name}\>*';
   }
 }