Update Object.assign signature. Fixes gh-954

Updates Object.assign to accept an arbitrary number of source objects whose own, enumerable property names and corresponding values are copied to the target. (Per April 9 TC39 consensus)

Signed-off-by: Rick Waldron <waldron.rick@gmail.com>
Avoid reassing module.exports on CommonJS modules

Replaced `module.exports = {...}` with `Object.defineProperties`.

Fixes #947
Report internal errors even when using Promises.

The part of traceur that uses promises is susceptiable to mystery errors.
Avoid some of this in InternalLoader.
Catch errors in inline-module loadNext() and report to console,

Review URL: https://api.github.com/repos/google/traceur-compiler/issues/952

Closes #952.
Reject fetch errors with Errors