Display potentially applicable extensions on nullable types (#3908)

Author: srawlins

lib/src/model/extension.dart

@@ -45,6 +45,7 @@ class Extension extends Container {
     if (extendedType is DynamicType || extendedType is VoidType) {
       return true;
     }
+    extendedType = library.element.typeSystem.promoteToNonNull(extendedType);
     var otherType = container.modelType.type;
     if (otherType is InterfaceType) {
       otherType = library.element.typeSystem.instantiateInterfaceToBounds(

test/templates/class_test.dart

@@ -314,6 +314,25 @@ extension E on C {
     ]);
   }
 
+  void test_instancePropertyAccessor_fromExtension_onNullableType() async {
+    await createPackageWithLibrary('''
+class C {}
+
+extension E on C? {
+  /// An instance getter.
+  int get f => 1;
+}
+''');
+    var htmlLines = readLines(['lib', 'C-class.html']);
+
+    htmlLines.expectMainContentContainsAllInOrder([
+      matches('<h2>Properties</h2>'),
+      matches('<dt id="f" class="property">'),
+      matches('<a href="../lib/E/f.html">f</a>'),
+      matches('An instance getter.'),
+    ]);
+  }
+
   void test_instancePropertyAccessor_fromExtensionOfSupertype() async {
     await createPackageWithLibrary('''
 class C<T> {}