fix(StateRegistry): Allow stateRegistry.register() to take a Ng1StateDeclaration

Closes #3345

Author: christopherthielen

src/interface.ts

@@ -671,4 +671,10 @@ export interface TemplateFactoryProvider {
    * @param useUnsafeHttpService `true` to use `$http` to fetch templates
    */
   useHttpService(useUnsafeHttpService: boolean);
+}
+
+declare module "ui-router-core/lib/state/stateRegistry" {
+  interface StateRegistry {
+    register(state: Ng1StateDeclaration)
+  }
 }