Avoid issues when calling methods on root package symbol

SymDenotations#topLevelClass: don't throw an exception
Symbols#associateFile: avoid infinite loop

Author: smarter

src/dotty/tools/dotc/core/SymDenotations.scala

@@ -868,7 +868,7 @@ object SymDenotations {
      */
     final def topLevelClass(implicit ctx: Context): Symbol = {
       def topLevel(d: SymDenotation): Symbol = {
-        if ((d is PackageClass) || (d.owner is PackageClass)) d.symbol
+        if (d.isEffectiveRoot || (d is PackageClass) || (d.owner is PackageClass)) d.symbol
         else topLevel(d.owner)
       }
       val sym = topLevel(this)

src/dotty/tools/dotc/core/Symbols.scala

@@ -517,7 +517,7 @@ object Symbols {
 
     /** The source or class file from which this class was generated, null if not applicable. */
     override def associatedFile(implicit ctx: Context): AbstractFile =
-      if (assocFile != null || (this.owner is PackageClass)) assocFile
+      if (assocFile != null || (this.owner is PackageClass) || this.isEffectiveRoot) assocFile
       else super.associatedFile
 
     final def classDenot(implicit ctx: Context): ClassDenotation =