code: function(fullName, options) {
          Ember.assert('fullName must be a proper full name', validateFullName(fullName));
          return lookup(this, this.normalize(fullName), options);
        }
docs: Given a fullName return a corresponding instance.
  
  The default behaviour is for lookup to return a singleton instance.
  The singleton is scoped to the container, allowing multiple containers
  to all have their own locally scoped singletons.
  
  ```javascript
  var container = new Container();
  container.register('api:twitter', Twitter);
  
  var twitter = container.lookup('api:twitter');
  
  twitter instanceof Twitter; // => true
  
  by default the container will return singletons
  var twitter2 = container.lookup('api:twitter');
  twitter2 instanceof Twitter; // => true
  
  twitter === twitter2; //=> true
  ```
  
  If singletons are not wanted an optional flag can be provided at lookup.
  
  ```javascript
  var container = new Container();
  container.register('api:twitter', Twitter);
  
  var twitter = container.lookup('api:twitter', { singleton: false });
  var twitter2 = container.lookup('api:twitter', { singleton: false });
  
  twitter === twitter2; //=> false
  ```
  
  @method lookup
  @param {String} fullName
  @param {Object} options
  @return {any}