[BUGFIX] Avoid String.prototype.repeat
[BUGFIX] Avoid String.prototype.repeat

We need to avoid this API because it is not supported in IE11, and Ember uses
these APIs in its tests. They are DEBUG APIs so it shouldn't affect end users,
but it's important to fix to land in Ember.

tildeio/glimmer,tildeio/glimmer,glimmerjs/glimmer-vm,tildeio/glimmer,glimmerjs/glimmer-vm,glimmerjs/glimmer-vm