Add experimental flexible types feature on top of explicit nulls

tests/flexible-types/run/array/Test.scala

@@ -0,0 +1,31 @@
+/*
+class Foo {
+  def err(msg: String): Nothing = {
+    throw new RuntimeException("Hello")
+  }
+  def retTypeNothing(): String = {
+    val y: String|Null = ???
+    if (y == null) err("y is null!")
+    y
+  }
+}
+*/
+
+
+object Test {
+  def main() = {
+    val i : Integer = new Integer(3) // Constructor with non-ref arg
+    val s1 : String | Null = new String("abc") // Constructor with ref arg
+    val s2 : String = new String("abc") // Constructor with ref arg, not null
+    val s3 = s1.nn.substring(0,1).substring(0,1)
+    val s4 = s2.substring(0,1).substring(0,1)
+    val s5 = s4.startsWith(s4)
+    // s1.substring(0,1) // error
+    val j : J = new J("")
+    println(s4)
+    //val f : Foo = new Foo("x")
+    //f.err("Hello")
+    //val l : List[String] = Java.returnsNull();
+    //val j : J = new J
+  }
+}