fix(key-manager): fix key manager test

Author: tinayuangao

src/cdk/a11y/key-manager/list-key-manager.spec.ts

@@ -30,6 +30,7 @@ class FakeQueryList<T> extends QueryList<T> {
   changes = new Subject<FakeQueryList<T>>();
   items: T[];
   get length() { return this.items.length; }
+  set length(_) { /* Empty setter for base class constructor */  }
   get first() { return this.items[0]; }
   toArray() { return this.items; }
   some() { return this.items.some.apply(this.items, arguments); }