function FilterObservable(source, predicate, thisArg) {
      this.source = source;
      this.predicate = bindCallback(predicate, thisArg, 3);
      __super__.call(this);
    }
Returns elements from an observable sequence as long as a specified condition is true.
 The element's index is used in the logic of the predicate function.
@param {Function} predicate A function to test each element for a condition; the second parameter of the function represents the index of the source element.
@param {Any} [thisArg] Object to use as this when executing callback.
@returns {Observable} An observable sequence that contains the elements from the input sequence that occur before the element at which the test no longer passes.