Revert "Check that we pickle a definition before its references"

More precisely, put it under a Config flag which is off by default.

This reverts commit ae1b00d.

Author: odersky

compiler/src/dotty/tools/dotc/core/tasty/TreePickler.scala

@@ -15,6 +15,7 @@ import Comments.{Comment, CommentsContext}
 import NameKinds._
 import StdNames.nme
 import transform.SymUtils._
+import config.Config
 import collection.mutable
 import dotty.tools.tasty.TastyFormat.ASTsSection
 
@@ -85,6 +86,11 @@ class TreePickler(pickler: TastyPickler) {
     case Some(label) =>
       if (label != NoAddr) writeRef(label) else pickleForwardSymRef(sym)
     case None =>
+      // See pos/t1957.scala for an example where this can happen.
+      // I believe it's a bug in typer: the type of an implicit argument refers
+      // to a closure parameter outside the closure itself. TODO: track this down, so that we
+      // can eliminate this case.
+      report.log(i"pickling reference to as yet undefined $sym in ${sym.owner}", sym.srcPos)
       pickleForwardSymRef(sym)
   }
 
@@ -197,7 +203,7 @@ class TreePickler(pickler: TastyPickler) {
       }
       else if (tpe.prefix == NoPrefix) {
         writeByte(if (tpe.isType) TYPEREFdirect else TERMREFdirect)
-        if !symRefs.contains(sym) && !sym.isPatternBound && !sym.hasAnnotation(defn.QuotedRuntimePatterns_patternTypeAnnot) then
+        if Config.checkLevels && !symRefs.contains(sym) && !sym.isPatternBound && !sym.hasAnnotation(defn.QuotedRuntimePatterns_patternTypeAnnot) then
           report.error(i"pickling reference to as yet undefined $tpe with symbol ${sym}", sym.srcPos)
         pickleSymRef(sym)
       }