Add alternative possition tracking template

When there is the need to recompile the Dotty library the assertion may fail.

Author: nicolasstucki

compiler/src/dotty/tools/dotc/ast/Positioned.scala

@@ -25,6 +25,7 @@ abstract class Positioned(implicit @constructorOnly src: SourceFile) extends Pro
 
   private def uniqueId_=(id: Int): Unit = {
     //assert(id != 2523, this)
+    // if (id == 1234) new Throwable().printStackTrace()
     myUniqueId = id
   }