assertCacheContents = (cachedAssets, expectedUrls) => {
    const expectedLength = expectedUrls.length;
    const cachedUrls = Object.keys(cachedAssets);
    const filteredUrls = cachedUrls.filter(url => expectedUrls.includes(url));
    filteredUrls.should.have.lengthOf(expectedLength);
    cachedUrls.should.have.lengthOf(expectedLength);
  }
Asserts that the keys in cachedAssets match exactly the list of expected URLs.
@param {Object} cachedAssets The result from a call to swUtils.getAllCachedAssets().
@param {Array.<String>} expectedUrls The expected cache contents.