function assertQuacksLike(obj, target) {
  for (const meth in target.prototype) {
    if (!googString.endsWith(meth, '_')) {
      assertNotUndefined(
          `Should have implemented ${meth}` +
              '()',
          obj[meth]);
    }
  }
}
Defined this way to preseve enumerability of the property for testing.

@return {string}