repl power mode improvements.

Implemented great suggestion from moors. More imports in power mode,
including the contents of treedsl. Also, another swing at overcoming
the mismatched global singletons problem, this time taking advantage of
dependent method types. Amazingly, it seems to work.

Continuing in the quest to create a useful compiler hacking environment,
there is now an implicit from Symbol which allows you to pretend a
Symbol takes type parameters, and the result is the applied type based
on the manifests of the type arguments and the type constructor of the
symbol. Examples:

    // magic with manifests
    scala> val tp = ArrayClass[scala.util.Random]
    tp: $r.global.Type = Array[scala.util.Random]

    // evidence
    scala> tp.memberType(Array_apply)
    res0: $r.global.Type = (i: Int)scala.util.Random

    // treedsl
    scala> val m = LIT(10) MATCH (CASE(LIT(5)) ==> FALSE, DEFAULT ==> TRUE)
    m: $r.treedsl.global.Match =
    10 match {
      case 5 => false
      case _ => true
    }

    // typed is in scope
    scala> typed(m).tpe
    res1: $r.treedsl.global.Type = Boolean

1 files changed, 4 insertions, 0 deletions

diff --git a/test/files/run/repl-power.scala b/test/files/run/repl-power.scala
index 9f70ac4b68..27da3df106 100644
--- a/test/files/run/repl-power.scala
+++ b/test/files/run/repl-power.scala
@@ -5,6 +5,10 @@ object Test extends ReplTest {
 :power
 // guarding against "error: reference to global is ambiguous"
 global.emptyValDef  // "it is imported twice in the same scope by ..."
+val tp = ArrayClass[scala.util.Random]    // magic with manifests
+tp.memberType(Array_apply)                // evidence
+val m = LIT(10) MATCH (CASE(LIT(5)) ==> FALSE, DEFAULT ==> TRUE) // treedsl
+typed(m).tpe                              // typed is in scope
   """.trim
 }