[no-master] Enable Scala 2.13.0 in the tools and all the other artifacts.

Partest is not yet enabled in this commit, because a dependency of
`partest` 2.13.0, namely `testkit`, was not published. See
scala/bug#11529 upstream.

Author: sjrd

js-envs-test-kit/src/main/scala/org/scalajs/jsenv/test/TimeoutTests.scala

@@ -140,7 +140,7 @@ trait TimeoutTests extends JSEnvTest {
       if (c >= 10)
         clearInterval(i);
     }, 10);
-    """ hasOutput (1 to 10).map(_ + "\n").mkString
+    """ hasOutput (1 to 10).mkString("", "\n", "\n")
 
     assertTrue("Execution took too little time", deadline.isOverdue())
 

js-envs/src/main/scala/org/scalajs/jsenv/ExternalJSEnv.scala

@@ -81,8 +81,18 @@ abstract class ExternalJSEnv(
      *  The default value in `ExternalJSEnv` is
      *  `System.getenv().asScala.toMap ++ env`.
      */
-    protected def getVMEnv(): Map[String, String] =
-      System.getenv().asScala.toMap ++ env
+    protected def getVMEnv(): Map[String, String] = {
+      /* We use Java's `forEach` not to depend on Scala's JavaConverters, which
+       * are very difficult to cross-compile across 2.12- and 2.13+.
+       */
+      val builder = Map.newBuilder[String, String]
+      System.getenv().forEach(new java.util.function.BiConsumer[String, String] {
+        def accept(key: String, value: String): Unit =
+        builder += key -> value
+      })
+      builder ++= env
+      builder.result()
+    }
 
     /** Get files that are a library (i.e. that do not run anything) */
     protected def getLibJSFiles(): Seq[VirtualJSFile] =

js-envs/src/main/scala/org/scalajs/jsenv/VirtualFileMaterializer.scala

@@ -77,7 +77,7 @@ final class VirtualFileMaterializer(singleDir: Boolean = false) {
   // scalastyle:on line.size.limit
   private def createTempDir(): File = {
     val baseDir = new File(System.getProperty("java.io.tmpdir"))
-    val baseName = System.currentTimeMillis() + "-"
+    val baseName = "" + System.currentTimeMillis() + "-"
 
     @tailrec
     def loop(tries: Int): File = {

js-envs/src/main/scala/org/scalajs/jsenv/nodejs/AbstractNodeJSEnv.scala

@@ -168,10 +168,18 @@ abstract class AbstractNodeJSEnv(
       val nodePath = libCache.cacheDir.getAbsolutePath +
           baseNodePath.fold("")(p => File.pathSeparator + p)
 
-      System.getenv().asScala.toMap ++ Seq(
-        "NODE_MODULE_CONTEXTS" -> "0",
-        "NODE_PATH" -> nodePath
-      ) ++ env
+      /* We use Java's `forEach` not to depend on Scala's JavaConverters, which
+       * are very difficult to cross-compile across 2.12- and 2.13+.
+       */
+      val builder = Map.newBuilder[String, String]
+      System.getenv().forEach(new java.util.function.BiConsumer[String, String] {
+        def accept(key: String, value: String): Unit =
+        builder += key -> value
+      })
+      builder += "NODE_MODULE_CONTEXTS" -> "0"
+      builder += "NODE_PATH" -> nodePath
+      builder ++= env
+      builder.result()
     }
   }