Ensure generated global MemberExpressions are not shared

[rwjblue]

The original fix in b63bb6e attempted to cache the generated member expressions, so that we didn't do duplicated work for each reference to a given global. Unfortunately, this optimization failed to take into consideration that most babel plugins work by directly mutating the node in question. In practice what that meant was that once any usage of a given global was needed to be transformed (e.g. say you had computed(...args, function(){}) and are transpiling for IE11), then all usages of that global would be mutated.

A more concrete example.

// input
import { computed } from '@ember/object';

function specialComputed(dependentKeys) {
  return computed(...dependentKeys, function() {});
}

function otherLessSpecialComputed() {
  return computed('stuff', 'hard', 'coded', function() {});
}

In this example, the first method (specialComputed) needs to be transpiled to something akin to (most of the changes here are from @babel/plugin-transform-spread):

function specialComputed(dependentKeys) {
  return Ember.computed.apply(Ember, dependentKeys.concat([function() {}]));
}

Unfortunately, since the generated MemberExpression for
Ember.computed is shared, this forced the other computed usage to be
transpiled to:

function otherLessSpecialComputed() {
  return Ember.computed.apply('stuff', 'hard', 'coded', function() {});
}

Which is clearly, totally invalid. 🤦