Could not find property 'dirs' on source set main.

[Jas]

I just wanted to write my first android instrumentation test. I added

sourceSets {
    main {
        dirs.each { dir ->
            java.srcDirs "src/${dir}/java"
            res.srcDirs "src/${dir}/res"
        }
    }
    androidTest.setRoot('tests')
    androidTest.java.srcDirs = ['tests/src']

}

same as I saw in the examples over:

/android-studio/sdk/samples/android-19/testing/ActivityInstrumentation/ActivityInstrumentationSample/build.gradle

In the example it looks fine (I'm able to see the tests folder colored in android studio) however in my project I get this error:

Error:(19, 0) Could not find property 'dirs' on source set main.

Not sure what is the different as both me and the example uses:

compileSdkVersion 19
buildToolsVersion "20.0.0"

http://stackoverflow.com/questions/24597680/how-to-use-standard-tests-directory-with-android-and-gradle 

(no answers here so posted to here as well)