Update Backbone collection.comparator definition to accpet string and Function types. Closes #6394.

backbone/backbone-global.d.ts

@@ -192,8 +192,10 @@ declare module Backbone {
 
         fetch(options?: CollectionFetchOptions): JQueryXHR;
 
-        comparator(element: TModel): number;
-        comparator(compare: TModel, to?: TModel): number;
+        /**
+         * Specify a model attribute name (string) or function that will be used to sort the collection.
+         */
+        comparator: string | ((element: TModel) => number | string) | ((compare: TModel, to?: TModel) => number);
 
         add(model: {}|TModel, options?: AddOptions): TModel;
         add(models: ({}|TModel)[], options?: AddOptions): TModel[];

backbone/backbone-tests.ts

@@ -120,6 +120,16 @@ class Library extends Backbone.Collection<Book> {
     // This model definition is here only to test type compatibility of the model, but it
     // is not necessary in working code as it is automatically inferred through generics.
     model: typeof Book;
+
+    constructor(models?: Book[] | Object[], options?: any) {
+        super(models, options);
+
+        // Test comparator allowed types.
+        this.comparator = "title";
+        this.comparator = (model: Book) => { return 1; };
+        this.comparator = (model: Book) => { return "Title"; };
+        this.comparator = (model1: Book, model2: Book) => { return 1; };
+    }
 }
 
 class Books extends Backbone.Collection<Book> { }