Allow staticElement to be null

Author: jcollins-g

lib/src/model/getter_setter_combo.dart

@@ -52,6 +52,8 @@ mixin GetterSetterCombo on ModelElement {
         .toString();
     Element staticElement =
         (constantInitializer as InstanceCreationExpression).staticElement;
+    if (staticElement == null)
+      return original;
     Constructor target = ModelElement.fromElement(staticElement, packageGraph);
     Class targetClass = target.enclosingElement;
     // TODO(jcollins-g): this logic really should be integrated into Constructor,