Mixin: fix the initialization of traits

Before this commit, the following code:
trait Hello {
  println("Hello")
  val x: Int = 1
  println("World")
}

Became:
<trait> trait Hello extends Object {
  def <init>(): Hello = {
    {
      ()
    }
    this
  }
  <accessor> def x(): Int
  protected def initial$x(): Int = {
    println("Hello")
    1
  }
}

Notice that the initialization statements after the last getter were
missing, this is now fixed:
<trait> trait Hello extends Object {
  def <init>(): Hello = {
    {
      println("World")
      ()
    }
    this
  }
  <accessor> def x(): Int
  protected def initial$x(): Int = {
    println("Hello")
    1
  }
}

1 files changed, 2 insertions, 0 deletions

diff --git a/tests/run/traitInit.check b/tests/run/traitInit.check
new file mode 100644
index 000000000..f9264f7fb
--- /dev/null
+++ b/tests/run/traitInit.check
@@ -0,0 +1,2 @@
+Hello
+World