Fix scala#22794: Emit the definition of Arrays.newArray even though it's a primitive.

Until 3.6.x, we used the user-space definition of `Arrays.newArray`
in Scala.js. We changed it to a primitve in
2852168, to match the behavior of
the JVM backend.

However, that had the side effect of not emitting its definition
at all, like other primitives. For backward binary compat reasons,
we do need that definition.

We specifically make an exception for that method, which is now
emitted, even though it is otherwise treated as a primitive.

[Cherry-picked 9c7193a]

Author: sjrd

compiler/src/dotty/tools/backend/sjs/JSCodeGen.scala

@@ -1496,7 +1496,7 @@ class JSCodeGen()(using genCtx: Context) {
 
       def jsParams = params.map(genParamDef(_))
 
-      if (primitives.isPrimitive(sym)) {
+      if (primitives.isPrimitive(sym) && sym != defn.newArrayMethod) {
         None
       } else if (sym.is(Deferred) && currentClassSym.isNonNativeJSClass) {
         // scala-js/#4409: Do not emit abstract methods in non-native JS classes

sbt-test/scalajs/backward-compat/build.sbt

@@ -0,0 +1,5 @@
+enablePlugins(ScalaJSPlugin)
+
+scalaVersion := sys.props("plugin.scalaVersion")
+
+libraryDependencies += "org.scalameta" %%% "munit" % "1.0.0-M10"

sbt-test/scalajs/backward-compat/project/plugins.sbt

@@ -0,0 +1,2 @@
+
+addSbtPlugin("org.scala-js" % "sbt-scalajs" % sys.props("plugin.scalaJSVersion"))

sbt-test/scalajs/backward-compat/src/test/scala/NewArrayIssue22794Test.scala

@@ -0,0 +1,5 @@
+class NewArrayIssue22794Test extends munit.FunSuite {
+  test("foo") {
+    assert(2 + 2 == 4)
+  }
+}

sbt-test/scalajs/backward-compat/test

@@ -0,0 +1 @@
+> test