@@can.getKeyDependencies(key)

The @@can.getKeyDependencies symbol points to a function that returns an object containing all values that affect the value of a computed property of this Map-like object, keyed on key.
If the dependencies include properties on a Map-like object, they are placed in a CIDMap keyed on keyDependencies in the return object. If the dependencies include the values of Value-like objects, they are placed in a CIDSet keyed on valueDependencies. Return undefined if the value is not computed, or an empty object if a key is computed without external dependencies.

var someOtherObj;
// This is a clear but not very useful implementation of a computed key value.
//  Map-likes in CanJS are backed by can-computes and can-observations, and we recommend can-observation for new development.
var obj = {
    _computedAttrs: {
        foo: [{ 
            handler: function() { return canReflect.getKeyValue(someOtherObj, "bar") },
            reads: [{ object: someOtherObj, key: "bar" }]
        }]
    }
};

obj[canSymbol.for("can.getKeyDependencies")] = function(key) {
  var ret;
  if(this._computedAttrs[key]) {
    ret = {};
    this._computedAttrs[key].reads.forEach(function(read) {
        if(read.key) {
            ret.keyDependencies = ret.keyDependencies || new CIDMap();
            if(!ret.keyDependencies.has(read.object)) {
                ret.keyDependencies.add(read.object, []);
            }
            ret.keyDependencies.get(read.object).push(read.key);
        } else {
            ret.valueDependencies = ret.valueDependencies || new CIDMap();
            ret.valueDependencies.add(read.object);
        }
    });
  }
  return ret;
};