Add extension operators plus and minus for FileCollection to make them resolve into gradle api, rather than collection api.

libraries/tools/kotlin-gradle-plugin/src/main/kotlin/org/jetbrains/kotlin/gradle/internal/AnnotationProcessingManager.kt

@@ -46,7 +46,7 @@ fun Project.initKapt(
         val stubsDir = File(getBuildDir(), "tmp/kapt/$variantName/classFileStubs")
         kotlinTask.extensions.extraProperties.set("kaptStubsDir", stubsDir)
 
-        javaTask.classpath = javaTask.classpath + files(stubsDir)
+        javaTask.classpath += files(stubsDir)
 
         kotlinTask.doFirst {
             kotlinAfterJavaTask.source(kotlinTask.source)
@@ -226,7 +226,7 @@ public class AnnotationProcessingManager(
     }
 
     private fun JavaCompile.appendClasspath(file: File) {
-        setClasspath(getClasspath() + project.files(file))
+        classpath += project.files(file)
     }
 
     private fun addWrappersToCompilerArgs(javaTask: JavaCompile, wrapperFqNames: String) {

libraries/tools/kotlin-gradle-plugin/src/main/kotlin/org/jetbrains/kotlin/gradle/plugin/KotlinPlugin.kt

@@ -25,6 +25,7 @@ import kotlin.properties.Delegates
 import org.gradle.api.tasks.Delete
 import groovy.lang.Closure
 import org.gradle.api.artifacts.Configuration
+import org.gradle.api.file.FileCollection
 import org.jetbrains.kotlin.gradle.tasks.KotlinTasksProvider
 import java.util.ServiceLoader
 import org.gradle.api.logging.*
@@ -538,6 +539,9 @@ open class GradleUtils(val scriptHandler: ScriptHandler, val project: ProjectInt
     public fun resolveJsLibrary(): File = resolveDependencies(kotlinJsLibraryCoordinates()).first()
 }
 
+internal operator fun FileCollection.plus(other: FileCollection) = this.plus(other)
+internal operator fun FileCollection.minus(other: FileCollection) = this.minus(other)
+
 fun AbstractCompile.storeKaptAnnotationsFile(kapt: AnnotationProcessingManager) {
     extensions.extraProperties.set("kaptAnnotationsFile", kapt.getAnnotationFile())
 }