Fix issue regarding WeakMap constructor

Author: Vimal-Raghubir

src/lib/es2015.collection.d.ts

@@ -30,8 +30,7 @@ interface WeakMap<K extends object, V> {
 }
 
 interface WeakMapConstructor {
-    new (): WeakMap<object, any>;
-    new <K extends object, V>(entries?: ReadonlyArray<[K, V]> | null): WeakMap<K, V>;
+    new <K extends object = object, V = any>(entries?: ReadonlyArray<[K, V]> | null): WeakMap<K, V>;
     readonly prototype: WeakMap<object, any>;
 }
 declare var WeakMap: WeakMapConstructor;