Don't let import symbols go stale

Otherwise we can run into problems when checking imports for e.g.
scala2Mode in later runs.

Author: odersky

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

@@ -41,7 +41,7 @@ trait SymDenotations { this: Context =>
   }
 
   def stillValid(denot: SymDenotation): Boolean =
-    if (denot.is(ValidForever) || denot.isRefinementClass) true
+    if (denot.is(ValidForever) || denot.isRefinementClass || denot.isImport) true
     else {
       val initial = denot.initial
       val firstPhaseId = initial.validFor.firstPhaseId.max(ctx.typerPhase.id)
@@ -590,6 +590,9 @@ object SymDenotations {
       originalName.isSetterName &&
       (!isCompleted || info.firstParamTypes.nonEmpty) // to avoid being fooled by   var x_= : Unit = ...
 
+    /** is this a symbol representing an import? */
+    final def isImport = name == nme.IMPORT
+
     /** is this the constructor of a class? */
     final def isClassConstructor = name == nme.CONSTRUCTOR
 

src/dotty/tools/dotc/printing/RefinedPrinter.scala

@@ -553,7 +553,7 @@ class RefinedPrinter(_ctx: Context) extends PlainPrinter(_ctx) {
       (sym.allOverriddenSymbols exists (_ is TypeParam))
 
   override def toText(sym: Symbol): Text = {
-    if (sym.name == nme.IMPORT) {
+    if (sym.isImport) {
       def importString(tree: untpd.Tree) = s"import ${tree.show}"
       sym.infoOrCompleter match {
         case info: Namer#Completer => return importString(info.original)