composer global require "fxp/composer-asset-plugin:1.0.0-beta3"

composer create-project --prefer-dist yiisoft/yii2-app-basic basic

The first command installs the composer asset plugin which allows managing bower and npm package dependencies through Composer. You only need to run this command once for all.

摘自：http://www.yiiframework.com/doc-2.0/guide-start-installation.html

你们别这样啊，解决了在这里说一下啊，我也遇到了这个问题，相信很多人都会遇到。。。