function testTwoMapLikeImplInterop(testImpl) {
  googIter.forEach(googIter.permutations(knownMapLikeImpls, 2), (p) => {
    testImpl(p[0], p[1]);
  });
}
For a given test implementation, this function calls the test implementation
once for every permutation (order-matters) of 2 of the well-known test
implementations. These tests are generally to ensure interoperability (e.g.
when constructing a new Map from the contents of an existing Map).
@param {function(!MapLikeCtor, !MapLikeCtor)} testImpl