Update non-ES6 packages to work properly with ES6 ones.

These additional package tests revealed a few more issues with the
exports from ember-metal/ember-runtime.
Do not attempt to delete/unassign the namespace name if undefined.

In strict mode calling `Ember.lookup[undefined] = undefined;` raises an
error. This changes simply moves that assignment into a guard to ensure
that `this.toString()` returns a valid value.