remove dependency on Gradle internal class

restores compatibility with Gradle 2.5

xtext-idea-gradle-plugin/src/main/java/org/xtext/gradle/idea/tasks/IdeaExtension.xtend

@@ -3,10 +3,10 @@ package org.xtext.gradle.idea.tasks
 import groovy.lang.Closure
 import java.io.File
 import java.util.List
+import java.util.concurrent.Callable
 import org.eclipse.xtend.lib.annotations.Accessors
 import org.gradle.api.Project
 import org.gradle.api.file.FileCollection
-import org.gradle.api.internal.file.collections.LazilyInitializedFileCollection
 
 import static extension org.xtext.gradle.idea.tasks.GradleExtensions.*
 
@@ -19,13 +19,13 @@ class IdeaExtension {
 	String ideaVersion
 	DownloadIdea downloadIdea
 	DownloadIdeaPlugins downloadPlugins
-	
+
 	new (Project project) {
 		this.project = project
 		pluginRepositories = new IdeaPluginRepositories
 		pluginDependencies = new IdeaPluginDependencies
 	}
-	
+
 	def List<Object> getIdeaLibs() {
 		val result = <Object>newArrayList
 		result += pluginDependencies.projectDependencies.map [
@@ -34,10 +34,10 @@ class IdeaExtension {
 		result.add(externalLibs)
 		result
 	}
-	
+
 	def FileCollection getExternalLibs() {
-		new LazilyInitializedFileCollection {
-			override createDelegate() {
+		project.files(new Callable<FileCollection> () {
+			override call() throws Exception {
 				val unpackedDependencies = pluginDependencies.externalDependencies.map[
 					pluginsCache / id / version
 				]
@@ -50,84 +50,82 @@ class IdeaExtension {
 				#[ideaCoreLibs, dependencyClasses, dependencyLibs].filterNull.reduce[a, b| a.plus(b)]
 			}
 
-			override getBuildDependencies() {
-				[#{downloadIdea, downloadPlugins}]
-			}
-		}
+		})
+		.builtBy(downloadIdea, downloadPlugins)
 	}
-	
+
 	def FileCollection getIdeaCoreLibs() {
 		project.fileTree(getIdeaHome + "/lib")
 			.builtBy(downloadIdea).include("*.jar") as FileCollection
 	}
-	
+
 	def FileCollection getIdeaRunClasspath() {
 		ideaCoreLibs.plus(toolsJar)
 	}
-	
+
 	def toolsJar() {
 		project.files('''«System.getProperty("java.home")»/../lib/tools.jar''')
 	}
-	
+
 	def File getIdeaHome() {
 		if (ideaHome == null) {
 			project.gradle.gradleUserHomeDir / "ideaSDK" / ideaVersion
 		} else {
-			project.file(ideaHome) 
+			project.file(ideaHome)
 		}
 	}
-	
+
 	def File getPluginsCache() {
 		project.gradle.gradleUserHomeDir / "ideaPluginDependencies"
 	}
-	
+
 	def File getSourcesZip() {
 		getIdeaHome / 'sources.zip'
 	}
-	
+
 	def File getSandboxDir() {
 		project.buildDir / "ideaSandbox"
 	}
-	
+
 	def pluginDependencies(Closure<Void> config) {
 		project.configure(pluginDependencies as Object, config)
 	}
-	
+
 	def pluginRepositories(Closure<Void> config) {
 		project.configure(pluginRepositories as Object, config)
 	}
 }
 
 class IdeaPluginRepositories implements Iterable<IdeaPluginRepository> {
 	val repositories = <IdeaPluginRepository>newTreeSet[$0.url.compareTo($1.url)]
-	
+
 	def void url(String url) {
 		repositories += new IdeaPluginRepository(url)
 	}
-	
+
 	override iterator() {
 		repositories.iterator
 	}
 }
 
 class IdeaPluginDependencies {
-	
+
 	val dependencies = <IdeaPluginDependency>newTreeSet[$0.id.compareTo($1.id)]
-	
+
 	def IdeaPluginDependency id(String id) {
 		val dependency = new IdeaPluginDependency(id)
 		dependencies += dependency
 		dependency
 	}
-	
+
 	def project(String id) {
 		id(id)
 	}
-	
+
 	def getExternalDependencies() {
 		dependencies.filter[version != null]
 	}
-	
+
 	def getProjectDependencies() {
 		dependencies.filter[version == null]
 	}
@@ -142,8 +140,8 @@ class IdeaPluginRepository {
 class IdeaPluginDependency {
 	val String id
 	String version
-	
+
 	def version(String version) {
 		this.version = version
 	}
-}
+}