Change the signature of Class's constructor to take an Object.

This way, the "public" signature of that constructor does not
refer to `ScalaJSClassData`, which is an implementation detail of
`java.lang.Class`. It is public because it is accessed by the
`Emitter`, even though it is declared as private.

Note that even though `ScalaJSClassData` as such is an
implementation detail, its *structure* must be considered as
specified at the interface between the `java.lang` implementation
and the linker. If someone wants to write a different linker, or a
different `java.lang` library, they must adhere to that structure
to ensure interoperability.

Author: sjrd

javalanglib/src/main/scala/java/lang/Class.scala

@@ -19,8 +19,9 @@ private trait ScalaJSClassData[A] extends js.Object {
   def newArrayOfThisClass(dimensions: js.Array[Int]): AnyRef = js.native
 }
 
-final class Class[A] private (private val data: ScalaJSClassData[A])
-    extends Object {
+final class Class[A] private (data0: Object) extends Object {
+  private val data: ScalaJSClassData[A] =
+    data0.asInstanceOf[ScalaJSClassData[A]]
 
   override def toString(): String = {
     (if (isInterface()) "interface " else

tools/shared/src/main/scala/org/scalajs/core/tools/linker/backend/emitter/Emitter.scala

@@ -675,7 +675,7 @@ private object Emitter {
           instantiateClass("sjsr_UndefinedBehaviorError", "init___T")
         },
 
-        instantiateClass("jl_Class", "init___jl_ScalaJSClassData"),
+        instantiateClass("jl_Class", "init___O"),
 
         instanceTests(LongImpl.RuntimeLongClass),
         instantiateClass(LongImpl.RuntimeLongClass, LongImpl.AllConstructors),