setAttribute方法
实现
@pragma('dart2js:tryInline')
void setAttribute(String name, Object value) {
// TODO(41258): Delete these assertions after forcing strong mode.
// Protect [name] against string conversion to "null" or "undefined".
assert(name != null, 'Attribute name cannot be null');
// TODO(sra): assert(value != null, 'Attribute value cannot be null.');
_setAttribute(name, value);
}