Determistically enter classes from directory into package scope

On Linux, the directory listing is not automatically sorted on Mac.
This leads to non-determistic ids of Symbols of the classes in a
directory, which in turn leads to instability of the ordering of
parents within inferred refinement types.

Notable, with this patch, we will stably infer:

```
scala> case class C(); case class D(); List(C(), D()).head
defined class C
defined class D
res0: Product with Serializable = C()
```

rather than sometimes getting `Serializable with Product` on
Linux. As such, I've removed the workarounds for this instability
in two test cases.

1 files changed, 21 insertions, 3 deletions

diff --git a/src/compiler/scala/tools/nsc/classpath/DirectoryClassPath.scala b/src/compiler/scala/tools/nsc/classpath/DirectoryClassPath.scala
index aba941e043..0d6925e8ac 100644
--- a/src/compiler/scala/tools/nsc/classpath/DirectoryClassPath.scala
+++ b/src/compiler/scala/tools/nsc/classpath/DirectoryClassPath.scala
@@ -5,6 +5,9 @@ package scala.tools.nsc.classpath
 
 import java.io.File
 import java.net.URL
+import java.util
+import java.util.Comparator
+
 import scala.reflect.io.{AbstractFile, PlainFile}
 import scala.tools.nsc.util.{ClassPath, ClassRepresentation}
 import FileUtils._
@@ -87,9 +90,24 @@ trait JFileDirectoryLookup[FileEntryType <: ClassRepresentation] extends Directo
     if (packageDir.exists && packageDir.isDirectory) Some(packageDir)
     else None
   }
-  protected def listChildren(dir: File, filter: Option[File => Boolean]): Array[File] = filter match {
-    case Some(f) => dir.listFiles(mkFileFilter(f))
-    case None => dir.listFiles()
+  protected def listChildren(dir: File, filter: Option[File => Boolean]): Array[File] = {
+    val listing = filter match {
+      case Some(f) => dir.listFiles(mkFileFilter(f))
+      case None => dir.listFiles()
+    }
+
+    // Sort by file name for stable order of directory .class entries in package scope.
+    // This gives stable results ordering of base type sequences for unrelated classes
+    // with the same base type depth.
+    //
+    // Notably, this will stably infer`Product with Serializable`
+    // as the type of `ase class C(); case class D(); List(C(), D()).head`, rather than the opposite order.
+    // On Mac, the HFS performs this sorting transparently, but on Linux the order is unspecified.
+    //
+    // Note this behaviour can be enabled with in javac with `javac -XDsortfiles`, but that's only
+    // intended to improve determinism of the compiler for compiler hackers.
+    util.Arrays.sort(listing, (o1: File, o2: File) => o1.getName.compareTo(o2.getName))
+    listing
   }
   protected def getName(f: File): String = f.getName
   protected def toAbstractFile(f: File): AbstractFile = new PlainFile(new scala.reflect.io.File(f))