The CompileOptions.bootClasspath property has been deprecated

After upgrading to Gradle 4.x, I get the warning

The CompileOptions.bootClasspath property has been deprecated and is scheduled to be removed in Gradle 5.0. Please use the CompileOptions.bootstrapClasspath property instead.

in one of my projects. I don't see anything called bootClasspath or similar in my build.gradle. What does this warning mean?

the warning only appears in the commons subproject, not in core.

commons/build.gradle:

apply plugin: 'com.android.library'

ext {
    PUBLISH_GROUP_ID = 'com.afollestad.material-dialogs'
    PUBLISH_ARTIFACT_ID = 'commons'
    PUBLISH_VERSION = '0.9.2.3'
    BUILD_TOOLS = "26.0.3"
    TARGET_SDK = 25
}

android {
    compileSdkVersion TARGET_SDK
    buildToolsVersion BUILD_TOOLS

    defaultConfig {
        minSdkVersion 16
        targetSdkVersion TARGET_SDK
        versionCode 1
        versionName PUBLISH_VERSION
    }
    lintOptions {
        checkReleaseBuilds false
    }
}

dependencies {
    implementation project(':core')
}

// Changes to this block must be applied in core/build.gradle and commons/build.gradle
task("javadoc", type: Javadoc) {
    description "Generates Javadoc API documentation for the main source code."
    source = android.sourceSets.main.java.srcDirs
    ext.androidJar = "${android.sdkDirectory}/platforms/${android.compileSdkVersion}/android.jar"
    classpath += files(ext.androidJar)
    exclude "**/BuildConfig.java"
    exclude "**/R.java"
    options.links("http://docs.oracle.com/javase/7/docs/api/");
    options.links("http://d.android.com/reference/");
}

core/build.gradle:

apply plugin: 'com.android.library'

ext {
    PUBLISH_GROUP_ID = 'com.afollestad.material-dialogs'
    PUBLISH_ARTIFACT_ID = 'core'
    PUBLISH_VERSION = '0.9.2.3'
    SUPPORT_LIBRARY_VERSION = '25.4.0'
    BUILD_TOOLS = "26.0.3"
    TARGET_SDK = 25
}

android {
    compileSdkVersion TARGET_SDK
    buildToolsVersion BUILD_TOOLS

    defaultConfig {
        minSdkVersion 16
        targetSdkVersion TARGET_SDK
        versionCode 1
        versionName PUBLISH_VERSION
        consumerProguardFiles 'progress-proguard.txt'
    }
    lintOptions {
        checkReleaseBuilds false
    }
}

dependencies {
    api "com.android.support:support-v13:$SUPPORT_LIBRARY_VERSION"
    api "com.android.support:appcompat-v7:$SUPPORT_LIBRARY_VERSION"
    api "com.android.support:recyclerview-v7:$SUPPORT_LIBRARY_VERSION"
    api "com.android.support:support-annotations:$SUPPORT_LIBRARY_VERSION"
    implementation "me.zhanghai.android.materialprogressbar:library:1.4.1"
}

// Changes to this block must be applied in core/build.gradle and commons/build.gradle
task("javadoc", type: Javadoc) {
    description "Generates Javadoc API documentation for the main source code."
    source = android.sourceSets.main.java.srcDirs
    ext.androidJar = "${android.sdkDirectory}/platforms/${android.compileSdkVersion}/android.jar"
    classpath += files(ext.androidJar)
    exclude "**/BuildConfig.java"
    exclude "**/R.java"
    options.links("http://docs.oracle.com/javase/7/docs/api/");
    options.links("http://d.android.com/reference/");
}