Fix #12621: Scala.js: Better error message for JS trait ctor param.

JS traits cannot have constructor params any more than they can
have concrete term members. This improves the error message in
that case.

Author: sjrd

Diff for: compiler/src/dotty/tools/dotc/transform/sjs/PrepJSInterop.scala

@@ -888,6 +888,9 @@ class PrepJSInterop extends MacroTransform with IdentityDenotTransformer { thisP
             report.error("A non-native JS trait cannot contain private members", tree)
           } else if (sym.is(Lazy)) {
             report.error("A non-native JS trait cannot contain lazy vals", tree)
+          } else if (sym.is(ParamAccessor)) {
+            // #12621
+            report.error("A non-native JS trait cannot have constructor parameters", tree)
           } else if (!sym.is(Deferred)) {
             /* Tell the back-end not to emit this thing. In fact, this only
              * matters for mixed-in members created from this member.

Diff for: tests/neg-scalajs/js-trait-ctor-param.check

@@ -0,0 +1,4 @@
+-- Error: tests/neg-scalajs/js-trait-ctor-param.scala:9:34 -------------------------------------------------------------
+9 |trait NonNativeBagHolderTrait(val bag: Bag) extends js.Any // error
+  |                              ^^^^^^^^^^^^
+  |                              A non-native JS trait cannot have constructor parameters

Diff for: tests/neg-scalajs/js-trait-ctor-param.scala

@@ -0,0 +1,9 @@
+import scala.scalajs.js
+import scala.scalajs.js.annotation.*
+
+@js.native
+trait Bag extends js.Any {
+  val str: String
+}
+
+trait NonNativeBagHolderTrait(val bag: Bag) extends js.Any // error