Cache keys are now tuples of (host, resource) so if you download and cache bootstrap.js on example.com, and then go to bootstrapsite.com which hosts the same JS file, you'll have to download it again because the cache keys are different:
example.com -> bootstrap.js -> cached as (example.com, bootstrap.js) bootstrapsite.com -> bootstrap.js -> cached as (bootstrapsite.com, bootstrap.js) (example.com, bootstrap.js) ≠ (bootstrapsite.com, bootstrap.js)
Cache keys are now tuples of (host, resource) so if you download and cache bootstrap.js on example.com, and then go to bootstrapsite.com which hosts the same JS file, you'll have to download it again because the cache keys are different: