Support extra module references in the REPL

By default, scala-compiler.jar is in the bootclasspath, which gives us a
`null` when we call `classOf[IMain].getClassLoader`. That value is used
as the parent class loader of the classloader that evals code in the REPL.

Under JDK9, this breaks lookup of classes on the module path:

```
 scala -J--add-modules=java.compiler -J--add-exports=jdk.jdeps/com.sun.tools.javap=ALL-UNNAMED
Welcome to Scala 2.12.1 (Java HotSpot(TM) 64-Bit Server VM, Java 9-ea).
Type in expressions for evaluation. Or try :help.

scala> new com.sun.tools.javap.JavapTask();
java.lang.NoClassDefFoundError: com/sun/tools/javap/JavapTask
  at java.base/java.lang.Class.getDeclaredMethods0(Native Method)
  at java.base/java.lang.Class.privateGetDeclaredMethods(Class.java:2948)
 ```

 A workaround is to use `-nobootcp`.

 This commit uses the system classloader as the parent in place of the null classloader
 to make this work in both cases.

 ```
 qscala -J--add-modules=java.compiler -J--add-exports=jdk.jdeps/com.sun.tools.javap=ALL-UNNAMED
 Welcome to Scala 2.12.2-20161208-165912-3de1c0c (Java HotSpot(TM) 64-Bit Server VM, Java 9-ea).
 Type in expressions for evaluation. Or try :help.

 scala> new com.sun.tools.javap.JavapTask();
 res0: com.sun.tools.javap.JavapTask = com.sun.tools.javap.JavapTask@1f1cddf3

 scala>
 ```

 Note that the `:javap` command still requires `-nobootcp` because code in the REPL implements
 an interface in a the `java.compiler` module.

2 files changed, 8 insertions, 4 deletions

diff --git a/src/repl/scala/tools/nsc/interpreter/ILoop.scala b/src/repl/scala/tools/nsc/interpreter/ILoop.scala
index 0dd96b2616..d1096fda4e 100644
--- a/src/repl/scala/tools/nsc/interpreter/ILoop.scala
+++ b/src/repl/scala/tools/nsc/interpreter/ILoop.scala
@@ -105,8 +105,10 @@ class ILoop(in0: Option[BufferedReader], protected val out: JPrintWriter)
   }
 
   class ILoopInterpreter extends IMain(settings, out) {
-    override protected def parentClassLoader =
-      settings.explicitParentLoader.getOrElse( classOf[ILoop].getClassLoader )
+    override protected def parentClassLoader = {
+      val replClassLoader = classOf[ILoop].getClassLoader // might be null if we're on the boot classpath
+      settings.explicitParentLoader.orElse(Option(replClassLoader)).getOrElse(ClassLoader.getSystemClassLoader)
+    }
   }
 
   /** Create a new interpreter. */
diff --git a/src/repl/scala/tools/nsc/interpreter/IMain.scala b/src/repl/scala/tools/nsc/interpreter/IMain.scala
index 99acc34811..980d12f9b8 100644
--- a/src/repl/scala/tools/nsc/interpreter/IMain.scala
+++ b/src/repl/scala/tools/nsc/interpreter/IMain.scala
@@ -255,8 +255,10 @@ class IMain(initialSettings: Settings, protected val out: JPrintWriter) extends
   }
 
   /** Parent classloader.  Overridable. */
-  protected def parentClassLoader: ClassLoader =
-    settings.explicitParentLoader.getOrElse( this.getClass.getClassLoader() )
+  protected def parentClassLoader: ClassLoader = {
+    val replClassLoader = this.getClass.getClassLoader() // might be null if we're on the boot classpath
+    settings.explicitParentLoader.orElse(Option(replClassLoader)).getOrElse(ClassLoader.getSystemClassLoader)
+  }
 
   /* A single class loader is used for all commands interpreted by this Interpreter.
      It would also be possible to create a new class loader for each command