(This was reverted by mistake. It had a bad commit message though, so relanding with a nicer one is nice anyhow.)
With this, we can do emsdk install latest-fastcomp and it installs fastcomp from the waterfall. That is, we then have 3 main sdks people might want to use:
* latest which installs fastcomp-llvm (plus emscripten etc.) from the mozilla infrastructure. (fetches the last emscripten version there)
* latest-upstream which installs upstream-llvm (plus emscripten etc.) from the waterfall infrastructure. (fetches the last known good revision (lkgr) there)
* latest-fastcomp which installs fastcomp-llvm (plus emscripten etc.) from the waterfall infrastructure. (fetches the last known good revision (lkgr) there)
The first and last are currently somewhat overlapping in that both fetch a build of fastcomp. However, as we transition away from the mozilla infrastructure, we could just make latest an alias for latest-fastcomp. (And later, when we're ready to switch to the wasm backend by default, the alias could switch to latest-upstream.)
This makes it possible to tell the emsdk to get "latest-upstream", which fetches the latest lkgr from there. This will probably be a common use pattern, I expect we may want to recommend users start trying out the wasm backend that way soon. This will also let us simplify this code: https://github.com/kripken/emscripten/blob/incoming/.circleci/config.yml#L334
Aside from adding the "latest-upstream" alias, this PR has
* A few minor cleanups in the emsdk code.
( Minor restructuring of how we define the upstream stuff in the manifest: It seemed odd to have 3 things (clang, emscripten, binaryen) that are all coming from a single archive from the waterfall. Simpler to have just one - the archive is one big lump, there's no way to download just part of it.
* Also add node 8.9.1 as a dependency of the upstream sdk, which makes things usable out of the box (node.js is the one thing not provided by the waterfall archive).
