Add test for pickled existential types bug (#616)

This test fails at the moment because there's one unnecessary
compile iteration performed. Thus the test is marked as pending.

Since the test is compiler version specific, I set it to 2.9.2
(latest stable version).

sbt/src/sbt-test/compiler-project/inc-pickled-existential/build.sbt

@@ -0,0 +1,12 @@
+logLevel := Level.Debug
+
+scalaVersion := "2.9.2"
+
+// dumps analysis into target/analysis-dump.txt file
+InputKey[Unit]("check-number-of-compiler-iterations") <<= inputTask { (argTask: TaskKey[Seq[String]]) =>
+  (argTask, compile in Compile) map { (args: Seq[String], a: sbt.inc.Analysis) =>
+    assert(args.size == 1)
+    val expectedIterationsNumber = args(0).toInt
+    assert(a.compilations.allCompilations.size == expectedIterationsNumber, "a.compilations.allCompilations.size = %d (expected %d)".format(a.compilations.allCompilations.size, expectedIterationsNumber))
+  }
+}

sbt/src/sbt-test/compiler-project/inc-pickled-existential/changes/B1.scala

@@ -0,0 +1,5 @@
+package test6
+
+class B extends A {
+  def wibble = println("fooo")
+}

sbt/src/sbt-test/compiler-project/inc-pickled-existential/pending

@@ -0,0 +1,12 @@
+# Tests if existential types are pickled correctly so they
+# do not introduce unnecessary compile iterations
+
+# introduces first compile iteration
+> compile
+# this change is local to a method and does not change the api so introduces
+# only one additional compile iteration
+$ copy-file changes/B1.scala src/main/scala/B.scala
+# second iteration
+> compile
+# check if there are only two compile iterations being performed
+> check-number-of-compiler-iterations 2

sbt/src/sbt-test/compiler-project/inc-pickled-existential/src/main/scala/A.scala

@@ -0,0 +1,9 @@
+package test6
+
+trait A {
+  object Foo extends Module[Foo[_]]
+
+  class Foo[TResult]
+
+  def b = new B
+}

sbt/src/sbt-test/compiler-project/inc-pickled-existential/src/main/scala/B.scala

@@ -0,0 +1,5 @@
+package test6
+
+class B extends A {
+  def wibble = println("foo")
+}

sbt/src/sbt-test/compiler-project/inc-pickled-existential/src/main/scala/Module.scala

@@ -0,0 +1,3 @@
+package test6
+
+class Module[T]