Fix JS DCE classpath duplicated in two input properties

* Do not add the classpath elements to source to avoid them being
  treated as task inputs without proper classpath normalization
* Move resolution of the classpath configuration to execution time

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

@@ -52,13 +52,12 @@ class KotlinJsDcePlugin : Plugin<Project> {
             val outputDir = File(File(project.buildDir, DEFAULT_OUT_DIR), sourceSet.name)
 
             val configuration = project.configurations.findByName(sourceSet.compileConfigurationName)!!
-            val dceInputTrees = listOf(project.fileTree(kotlinTask.outputFile)) + configuration.map { project.fileTree(it) }
-            val dceInputFiles = dceInputTrees.reduce { acc: FileTree, tree -> acc + tree }
+            val dceInputTree = project.fileTree(kotlinTask.outputFile)
 
             with (dceTask) {
-                classpath = sourceSet.compileClasspath
+                classpath = configuration
                 destinationDir = dceTask.dceOptions.outputDirectory?.let { File(it) } ?: outputDir
-                source(dceInputFiles)
+                source(dceInputTree)
             }
         }
     }

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

@@ -17,13 +17,13 @@
 package org.jetbrains.kotlin.gradle.tasks
 
 import org.gradle.api.Project
+import org.gradle.api.file.FileTree
 import org.gradle.api.tasks.CacheableTask
 import org.gradle.api.tasks.Input
 import org.gradle.api.tasks.Internal
 import org.gradle.api.tasks.TaskAction
 import org.jetbrains.kotlin.cli.common.arguments.K2JSDceArguments
 import org.jetbrains.kotlin.cli.js.dce.K2JSDce
-import org.jetbrains.kotlin.compilerRunner.ArgumentUtils
 import org.jetbrains.kotlin.compilerRunner.GradleKotlinLogger
 import org.jetbrains.kotlin.compilerRunner.createLoggingMessageCollector
 import org.jetbrains.kotlin.compilerRunner.runToolInSeparateProcess
@@ -61,7 +61,9 @@ open class KotlinJsDce : AbstractKotlinCompileTool<K2JSDceArguments>(), KotlinJs
 
     @TaskAction
     fun performDce() {
-        val inputFiles = getSource().files.map { it.path }
+        val inputFiles = (listOf(getSource()) + classpath.map { project.fileTree(it) })
+                .reduce(FileTree::plus)
+                .files.map { it.path }
 
         val outputDirArgs = arrayOf("-output-dir", destinationDir.path)