Fix #22794: Emit the definition of Arrays.newArray even though it's a primitive. (#22797)

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.

Author: sjrd

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

@@ -1557,7 +1557,7 @@ 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" % "1.1.0"

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