Allow class extensions to define which static properties to merge onto the built class from Y.Base.create/mix

A_and_B_ext.js

// See ticket #2531567

// Use the current support for SuperClass._buildCfg.custom.{anyName} = handlerFunction to look for
// a new property Ext._buildCfg.buildFn on each class extension. If there, execute it. The extension
// then uses that function to manually migrate its static properties of interest.

// Two approaches are shown, but both rely on the Ext._buildCfg.buildFn static.  This property and
// name are arbitrary.  What matters is that the handlerFunction assigned to the superclass's
// _buildCfg.custom collection establishes the contract for where to find this attribute on each
// class extension.

// Two extensions for example purposes
function A() {}
A.staticFromA = function () { console.log('Static from A'); };
A._buildCfg = {
    buildFn: function (HostClass, config) {
        console.log('Ext A buildFn');

        // Manually migrate a static property supplied by the extension (this)
        HostClass.staticFromA = this.staticFromA;
    }
};

function B() {}
B.staticFromB = function () { console.log('Static from B'); };
B._buildCfg = {
    buildFn: function (HostClass, config) {
        console.log('Ext B buildFn');

        // Manually migrate a static property supplied by the extension (this)
        HostClass.staticFromB = this.staticFromB;
    }
};

C_with_Base.js

// Approach #1, Add the new custom aggregator to Y.Base directly.
// CAVEAT: This will add a tiny bit of work to every call to Base.create and Base.mix
Y.Base._buildCfg.custom._ext = function (_, HostClass, Ext) {
    var config  = Ext._buildCfg,
        buildFn = config && config.buildFn;

    buildFn && buildFn.call(Ext, HostClass, config);
};

var C = Y.Base.create('c', Y.Base, [A, B]);

// Also works with Base.create('c', Y.Base, [A]); Base.mix(C, [B]);

C.staticFromA();
C.staticFromB();

var c = new C();

C_with_intermediate.js

// Approach #2, insert an intermediate subclass of Base that includes the new custom aggregator:
var Intermediate = Y.extend(
    function () { Y.Base.apply(this, arguments); },
    Y.Base,
    null, {
        _buildCfg: {
            custom: {
                _ext: function (_, HostClass, Ext) {
                    var config  = Ext._buildCfg,
                        buildFn = config && config.buildFn;

                    buildFn && buildFn.call(Ext, HostClass, config);
                }
            }
        }
    });

var C = Y.Base.create('c', Intermediate, [A, B]);

// Also works with Base.create('c', Y.Base, [A]); Base.mix(C, [B]);

C.staticFromA();
C.staticFromB();

var c = new C();

Thanks for digging in.