Change check logic to address reviewer's comments

Author: odersky

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

@@ -112,9 +112,9 @@ object Trees {
      *  type. (Overridden by empty trees)
      */
     def withType(tpe: Type)(implicit ctx: Context): ThisTree[Type] = {
-      if (Config.checkUnreportedErrors)
-        if (tpe.isInstanceOf[ErrorType]) assert(ctx.reporter.errorsReported)
-      if (Config.checkTreesConsistent)
+      if (tpe.isInstanceOf[ErrorType])
+        assert(!Config.checkUnreportedErrors || ctx.reporter.errorsReported)
+      else if (Config.checkTreesConsistent)
         checkChildrenTyped(productIterator)
       withTypeUnchecked(tpe)
     }