vue兼容ie8的解決方案，解決IE8下不支持document.getElementsByClassName的方法

在代碼前面加如下代碼：

if (!document.getElementsByClassName) {
document.getElementsByClassName = function (className, element) {
var children = (element || document).getElementsByTagName('*');
var elements = new Array();
for (var i = 0; i < children.length; i++) {
var child = children[i];
var classNames = child.className.split(' ');
for (var j = 0; j < classNames.length; j++) {
if (classNames[j] == className) {
elements.push(child);
break;
}
}
}
return elements;
};
}