SI-7747 Make REPL wrappers serialization friendly

Spark has been shipping a forked version of our REPL for
sometime. We have been trying to fold the patches back into
the mainline so they can defork. This is the last outstanding
issue.

Consider this REPL session:

```
scala> val x = StdIn.readInt

scala> class A(a: Int)

scala> serializedAndExecuteRemotely {
  () => new A(x)
}
```

As shown by the enclosed test, the REPL, even with the
Spark friendly option `-Yrepl-class-based`, will re-initialize
`x` on the remote system.

This test simulates this by running a REPL session, and then
deserializing the resulting closure into a fresh classloader
based on the class files generated by that session. Before this
patch, it printed "evaluating x" twice.

This is based on the Spark change described:

  https://github.com/mesos/spark/pull/535#discussion_r3541925

A followup commit will avoid the `val lineN$read = ` part if we
import classes or type aliases only.

[Original commit from Prashant Sharma, test case from Jason Zaugg]

1 files changed, 23 insertions, 9 deletions

diff --git a/test/files/run/t7747-repl.check b/test/files/run/t7747-repl.check
index 105b238d01..6eb8c8cd99 100644
--- a/test/files/run/t7747-repl.check
+++ b/test/files/run/t7747-repl.check
@@ -112,7 +112,7 @@ scala> 55 ; ((2 + 2)) ; (1, 2, 3)
 res15: (Int, Int, Int) = (1,2,3)
 
 scala> 55 ; (x: Int) => x + 1 ; () => ((5))
-<console>:8: warning: a pure expression does nothing in statement position; you may be omitting necessary parentheses
+<console>:9: warning: a pure expression does nothing in statement position; you may be omitting necessary parentheses
               55 ; (x: Int) => x + 1 ;;
               ^
 res16: () => Int = <function0>
@@ -258,12 +258,15 @@ class $read extends Serializable {
       super.<init>;
       ()
     };
-    import $line44.$read.$iw.$iw.BippyBups;
-    import $line44.$read.$iw.$iw.BippyBups;
-    import $line45.$read.$iw.$iw.PuppyPups;
-    import $line45.$read.$iw.$iw.PuppyPups;
-    import $line46.$read.$iw.$iw.Bingo;
-    import $line46.$read.$iw.$iw.Bingo;
+    val $line44$read = $line44.$read.INSTANCE;
+    import $line44$read.$iw.$iw.BippyBups;
+    import $line44$read.$iw.$iw.BippyBups;
+    val $line45$read = $line45.$read.INSTANCE;
+    import $line45$read.$iw.$iw.PuppyPups;
+    import $line45$read.$iw.$iw.PuppyPups;
+    val $line46$read = $line46.$read.INSTANCE;
+    import $line46$read.$iw.$iw.Bingo;
+    import $line46$read.$iw.$iw.Bingo;
     class $iw extends Serializable {
       def <init>() = {
         super.<init>;
@@ -275,12 +278,23 @@ class $read extends Serializable {
   };
   val $iw = new $iw.<init>
 }
-object $read extends $read {
+object $read extends scala.AnyRef {
   def <init>() = {
     super.<init>;
     ()
-  }
+  };
+  val INSTANCE = new $read.<init>
 }
 res3: List[Product with Serializable] = List(BippyBups(), PuppyPups(), Bingo())
 
+scala> :power
+** Power User mode enabled - BEEP WHIR GYVE **
+** :phase has been set to 'typer'.          **
+** scala.tools.nsc._ has been imported      **
+** global._, definitions._ also imported    **
+** Try  :help, :vals, power.<tab>           **
+
+scala> intp.lastRequest
+res4: $r.intp.Request = Request(line=def $ires3 = intp.global, 1 trees)
+
 scala> :quit