Use .js for module names

Issue #1221
Support type assertions for es6 output

fix #1442
Move test-compat-table out of make test.

Remove the git url in the package.json and put it in the make rule.
The automatic test force network access on every make test and
added more dependencies than value we got from automatic testing.
(The current data file from compat-table fails in V8 due to
SyntaxError: Duplicate data property in object literal not allowed in strict mode

Review URL: https://api.github.com/repos/google/traceur-compiler/issues/1501

Closes #1501.
Report a clear error if the traceur runtime is missing when

For the case where generators.js runs without a runtime.js first.
Fixes #1504

Review URL: https://api.github.com/repos/google/traceur-compiler/issues/1505

Closes #1505, closes #1504.
Split System.register into .register() and .registerModule().
'instantiate' -> .register()
'register' -> .registerModule()
This allows the two formats to evolve independently.