It looks like SystemJS 2.0 is a super-minimal rewrite, for which features can be added at build time. I don't really like the removal of support for bundling. The justification is "just compile multiple files serverside, or just makes lots of requests and http/2 will pick up the slack". I wish I could use http/2, but with IT, old servers, and clients with IT and managed workstations full of legacy software, plenty of important machines run Windows 7 (or equivalent) with no http/2 support. As for compiling server side, if I create a build containing Backbone and dependencies, and later, I request underscore, there is no way to know that underscore has already been loaded -- which takes away a great deal of dynamism.