Revert a tiny recent refactoring in SelectiveCPSTransform

The refactoring in 7e6c723df means that we can't build the CPS
plugin if we skip locker in development mode.

This commit backs out the refactoring and leaves a TODO comment
to perform it at a later date.

Author: retronym

src/continuations/plugin/scala/tools/selectivecps/SelectiveCPSTransform.scala

@@ -330,6 +330,10 @@ abstract class SelectiveCPSTransform extends PluginComponent with
                 }
               }
 
+              // TODO use gen.mkBlock after 2.11.0-M6. Why wait? It allows us to still build in development
+              // mode with `ant -DskipLocker=1`
+              def mkBlock(stms: List[Tree], expr: Tree) = if (stms.nonEmpty) Block(stms, expr) else expr
+
               try {
                 if (specialCaseTrivial) {
                   debuglog("will optimize possible tail call: " + bodyExpr)
@@ -348,17 +352,17 @@ abstract class SelectiveCPSTransform extends PluginComponent with
                   val argSym = currentOwner.newValue(vd.symbol.name.toTermName).setInfo(tpe)
                   val argDef = localTyper.typed(ValDef(argSym, Select(ctxRef, ctxRef.tpe.member(cpsNames.getTrivialValue))))
                   val switchExpr = localTyper.typedPos(vd.symbol.pos) {
-                    val body2 = gen.mkBlock(bodyStms :+ bodyExpr).duplicate // dup before typing!
+                    val body2 = mkBlock(bodyStms, bodyExpr).duplicate // dup before typing!
                     If(Select(ctxRef, ctxSym.tpe.member(cpsNames.isTrivial)),
-                      applyTrivial(argSym, gen.mkBlock((argDef :: bodyStms) :+ bodyExpr)),
+                      applyTrivial(argSym, mkBlock(argDef::bodyStms, bodyExpr)),
                       applyCombinatorFun(ctxRef, body2))
                   }
                   (List(ctxDef), switchExpr)
                 } else {
                   // ctx.flatMap { <lhs> => ... }
                   //     or
                   // ctx.map { <lhs> => ... }
-                  (Nil, applyCombinatorFun(rhs1, gen.mkBlock(bodyStms :+ bodyExpr)))
+                  (Nil, applyCombinatorFun(rhs1, mkBlock(bodyStms, bodyExpr)))
                 }
               } catch {
                 case ex:TypeError =>