const Mixin = bemDom.declMixin({ myProp : 'test' });
Block = bemDom.declBlock('block', [Mixin]);

Block.prototype.findChildBlock // undefined

const Mixin = bemDom.declMixin({ myProp : 'test' });
Block = bemDom.declBlock('block', [bemDom.Block, Mixin]);

Block.prototype.findChildBlock // function

    /**
     * Declares DOM-based block and creates block class
     * @param {String|Function} blockName Block name or block class
     * @param {Function|Array[Function]} [base] base block + mixes
     * @param {Object} [props] Methods
     * @param {Object} [staticProps] Static methods
     * @returns {Function} Block class
     */
    declBlock : function(blockName, base, props, staticProps) {
        if(!base || (typeof base === 'object' && !Array.isArray(base))) {
            staticProps = props;
            props = base;
            base = typeof blockName === 'string'?
                entities[blockName] || Block :
                blockName;
        }

        // added code start
        base || (base = entities[blockName] || Block);

        Array.isArray(base) || (base = [base]);

        if(!base[0].__bemEntity) {
            base = base.slice();
            base.unshift(entities[blockName] || Block);
        }
         // added code end

        return bem.declBlock(blockName, base, props, staticProps);
    },

Block = bemDom.declBlock('block', [Mixin]);
Block.prototype.findChildBlock // function