问题如下：
[2018-01-15 11:46:26 - Dex Loader] Failed to load F:\Android-SDK\build-tools\26.0.1\lib\dx.jar
[2018-01-15 11:46:26 - HelloAndroid] Unknown error: Unable to build: the file dx.jar was not loaded from the SDK folder![2018-01-15 11:49:09 - Dex Loader] Failed to load F:\Android-SDK\build-tools\26.0.1\lib\dx.jar
[2018-01-15 11:49:09 - HelloAndroid] Unknown error: Unable to build: the file dx.jar was not loaded from the SDK folder!
[2018-01-15 11:49:40 - Dex Loader] Failed to load F:\Android-SDK\build-tools\26.0.1\lib\dx.jar
[2018-01-15 11:49:40 - HelloAndroid] Unknown error: Unable to build: the file dx.jar was not loaded from the SDK folder!
[2018-01-15 11:50:15 - Dex Loader] Failed to load F:\Android-SDK\build-tools\26.0.1\lib\dx.jar
[2018-01-15 11:50:15 - HelloAndroid] Unknown error: Unable to build: the file dx.jar was not loaded from the SDK folder!
[2018-01-15 11:55:34 - Dex Loader] Failed to load F:\Android-SDK\build-tools\26.0.1\lib\dx.jar
[2018-01-15 11:55:34 - HelloAndroid] Unknown error: Unable to build: the file dx.jar was not loaded from the SDK folder!
[2018-01-15 11:56:50 - Dex Loader] Failed to load F:\Android-SDK\build-tools\26.0.1\lib\dx.jar
[2018-01-15 11:56:50 - HelloAndroid] Unknown error: Unable to build: the file dx.jar was not loaded from the SDK folder!
都说找不到26.0.1这个版本的sdk里lib文件夹下的dx.jar文件。
解决办法：把早期版本的sdk（如25.0.2）的lib文件夹里的dx.jar文件粘贴并替换掉他报错中的这个出错的dx.jar