function abort() {
        expect(this).toBe(innerOperation);
        abortCalled = true;
        return Promise.resolve();
      }
NOTE: This is a subtle thing, but this must be an ES5 anonymous
function for the test to work.  ES6 arrow functions would always be
called with the "this" of the test itself, regardless of what the
library is doing.

@this {shaka.util.AbortableOperation}
@return {!Promise}