* If the mutability of the global object doesn't match the module, then
  it should throw a LinkError.
* There was a missing `return` when importing a Number as a mutable
  global.
* All globals were being exported as immutable.
* Attempting to set the value of an immutable global should throw a
  TypeError.
* The length of the setter function should be 1.

Bug: v8:7625
Change-Id: I08d6a428506a18db15eecadf4cbcee89e0658924
Reviewed-on: https://chromium-review.googlesource.com/1031626Reviewed-by: Michael Starzinger <mstarzinger@chromium.org>
Commit-Queue: Ben Smith <binji@chromium.org>
Cr-Commit-Position: refs/heads/master@{#52865}