Make doc work with sbt 1.4.x

Scaladoc are not generated with 1.4.0-M2:
sbt/sbt#5798. There seemed to be three problems:
1. the sources were not actually specified in ScaladocBridge
2. Three was an assumption that the compiler would return only
   AbstractZincFiles in DelegatingReporter.makePosition but, in doc at
   least, it returns scala.reflect.io.PlainFile

These are straightforward to fix though I am somewhat concerned that the
pattern match in DelegatingReporter has an unhandled case.

internal/compiler-bridge/src/main/scala/xsbt/DelegatingReporter.scala

@@ -17,6 +17,7 @@ import java.util.Optional
 import scala.reflect.internal.util.{ FakePos, NoPosition, Position }
 // Left for compatibility
 import Compat._
+import scala.reflect.io.PlainFile
 
 private object DelegatingReporter {
   def apply(settings: scala.tools.nsc.Settings, delegate: xsbti.Reporter): DelegatingReporter =
@@ -89,7 +90,10 @@ private object DelegatingReporter {
 
     def makePosition(pos: Position): xsbti.Position = {
       val src = pos.source
-      val sourcePath = src.file match { case AbstractZincFile(virtualFile) => virtualFile.id }
+      val sourcePath = src.file match {
+        case AbstractZincFile(virtualFile) => virtualFile.id
+        case f: PlainFile                  => f.file.toString
+      }
       val sourceFile = new File(src.file.path)
       val line = pos.line
       val lineContent = pos.lineContent.stripLineEnd

internal/compiler-bridge/src/main/scala/xsbt/ScaladocBridge.scala

@@ -30,12 +30,13 @@ private class Runner(
   import scala.tools.nsc.{ doc, Global, reporters }
   import reporters.Reporter
   val docSettings: doc.Settings = new doc.Settings(Log.settingsError(log))
-  val command = new CompilerCommand(args.toList, docSettings)
+  val fullArgs = args.toList ++ sources.map(_.toString)
+  val command = new CompilerCommand(fullArgs, docSettings)
   val reporter = DelegatingReporter(docSettings, delegate)
   def noErrors = !reporter.hasErrors && command.ok
 
   def run(): Unit = {
-    debug(log, "Calling Scaladoc with arguments:\n\t" + args.mkString("\n\t"))
+    debug(log, "Calling Scaladoc with arguments:\n\t" + fullArgs.mkString("\n\t"))
     if (noErrors) {
       import doc._ // 2.8 trunk and Beta1-RC4 have doc.DocFactory.  For other Scala versions, the next line creates forScope.DocFactory
       val processor = new DocFactory(reporter, docSettings)