Caching libraries

[rickmcgeer]

Describe the Feature
Freezing has been failing for me this afternoon because

TypeError: Could not load https://jspm.dev/npm:immutable@4.0.0-rc.12!cjs: fetch failed

Twice it was due to a Connect Timeout Error, and once due to a SocketError: other side closed.

Presumably due to some outage at jspm.dev, or jspm.dev got too budy, or...I will try again later.
But it did make me wonder why the freezer doesn't download the node_modules (as node applications do) and use the cached version of the library rather than fetching on every build.

Graphics
If applicable, provide screenshots/videos of similar ideas in other software.

Implementation Details
If applicable, provide insights with regards to implementing this feature that you already have.

[merryman]

It should download them locally and then no longer bother. We do support offline working with lively.next after all. Is this a clean install?