js interop design

[jmesserly]

JS interop needs some design work.

Ideally we'd support "header files" similar to TypeScript. This would provide tooling feedback on usage of JS libraries.

Naturally, we'll also need to support dynamic calls into JavaScript, where no annotations are available.

[jmesserly]

the design here has been done and is now supported by dart:js (sharing most features with dart2js, dartium)