TreeChecker#adapt: do not check PolyProto

This should be safe like skipping FunProto since we'll check the type
application result anyway. This is needed to get -Ycheck:frontend to
work when named type parameters are used like in t1513a.scala

Author: smarter

compiler/src/dotty/tools/dotc/transform/TreeChecker.scala

@@ -443,7 +443,8 @@ class TreeChecker extends Phase with SymTransformer {
       if (ctx.mode.isExpr &&
           !tree.isEmpty &&
           !isPrimaryConstructorReturn &&
-          !pt.isInstanceOf[FunProto])
+          !pt.isInstanceOf[FunProto] &&
+          !pt.isInstanceOf[PolyProto])
         assert(tree.tpe <:< pt, {
           val mismatch = err.typeMismatchMsg(tree.tpe, pt)
           i"""|${mismatch.msg}