找不到渐变中的复制任务

我四处寻找如何做到这一点,这就是我创造的(我消除了所有的“噪音”):

buildscript {
    repositories {
        jcenter()
    }

    dependencies {
        classpath 'com.android.tools.build:gradle:2.3.0'
    }
}

allprojects {
   repositories {
      flatDir {
        dirs 'libs'
      }
   }
}

apply plugin: 'com.android.application'

dependencies {
    compile fileTree(dir: 'libs', include: ['*.jar'])
}

android {

    compileSdkVersion  25
      .....
    buildTypes {
     .......
    }

     preBuild.dependsOn copyRes
}

task copyRes(type: Copy) {
    from file("'../../Assets/Plugins/Android/res")
    into file("./src/main/res/values")
}

生成失败,错误如下:

Could not get unknown property 'copyRes' for object of type com.android.build.gradle.AppExtension