Add a customInitFiles() hook in ExternalJSEnv.

This will simplify the user-space workaround for scala-js/scala-js#1555. It can also
be generally useful.

Author: sjrd

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

@@ -22,13 +22,20 @@ abstract class ExternalJSEnv(
   /** Command to execute (on shell) for this VM */
   protected def executable: String
 
+  /** Custom initialization scripts. */
+  protected def customInitFiles(): Seq[VirtualJSFile] = Nil
+
   protected class AbstractExtRunner(protected val classpath: CompleteClasspath,
       protected val code: VirtualJSFile, protected val logger: Logger,
       protected val console: JSConsole) {
 
     /** JS files used to setup VM */
     protected def initFiles(): Seq[VirtualJSFile] = Nil
 
+    /** Custom initialization scripts, defined by the environment. */
+    final protected def customInitFiles(): Seq[VirtualJSFile] =
+      ExternalJSEnv.this.customInitFiles()
+
     /** Sends required data to VM Stdin (can throw) */
     protected def sendVMStdin(out: OutputStream): Unit = {}
 
@@ -46,7 +53,7 @@ abstract class ExternalJSEnv(
 
     /** Get files that are a library (i.e. that do not run anything) */
     protected def getLibJSFiles(): Seq[VirtualJSFile] =
-      initFiles() ++ classpath.allCode
+      initFiles() ++ customInitFiles() ++ classpath.allCode
 
     /** Get all files that are passed to VM (libraries and code) */
     protected def getJSFiles(): Seq[VirtualJSFile] =

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

@@ -317,6 +317,7 @@ class NodeJSEnv private (
     /** Libraries are loaded via require in Node.js */
     override protected def getLibJSFiles(): Seq[VirtualJSFile] = {
       initFiles() ++
+      customInitFiles() ++
       classpath.jsLibs.map(requireLibrary) :+
       classpath.scalaJSCode
     }

js-envs/src/test/scala/org/scalajs/jsenv/test/CustomInitFilesTest.scala

@@ -0,0 +1,40 @@
+package org.scalajs.jsenv.test
+
+import org.scalajs.jsenv._
+import org.scalajs.jsenv.nodejs.NodeJSEnv
+import org.scalajs.jsenv.phantomjs.PhantomJSEnv
+
+import org.scalajs.core.tools.io._
+
+import org.junit.Test
+
+abstract class CustomInitFilesTest extends JSEnvTest {
+  def makeCustomInitFiles(): Seq[VirtualJSFile] = {
+    Seq(new MemVirtualJSFile("custominit.js").withContent("""
+      function customPrint(s) {
+        console.log("custom: " + s);
+      }
+    """))
+  }
+
+  @Test
+  def customInitFilesTest = {
+    """
+    customPrint("hello");
+    """ hasOutput
+    """|custom: hello
+       |""".stripMargin
+  }
+}
+
+class NodeJSWithCustomInitFilesTest extends CustomInitFilesTest {
+  protected def newJSEnv = new NodeJSEnv {
+    override def customInitFiles() = makeCustomInitFiles()
+  }
+}
+
+class PhantomJSWithCustomInitFilesTest extends CustomInitFilesTest {
+  protected def newJSEnv = new PhantomJSEnv {
+    override def customInitFiles() = makeCustomInitFiles()
+  }
+}