Add additional flow typing tests

One tests interaction with singleton types; the other with implicits.
Fengyun and Alexander suggested these.

Author: abeln

tests/explicit-nulls/neg/flow-implicitly.scala

@@ -0,0 +1,10 @@
+
+// Test that flow typing works well with implicit resolution.
+class Test {
+  implicit val x: String | Null = ???
+  implicitly[x.type <:< String] // error: x.type is widened String|Null
+
+  if (x != null) {
+    implicitly[x.type <:< String] // ok: x.type is widened to String
+  }
+}

tests/explicit-nulls/pos/flow-singleton.scala

@@ -0,0 +1,9 @@
+// Test that flow typing works well with singleton types.
+
+class Test {
+  val x : String | Null = ???
+  if (x != null) {
+    val y: x.type = x
+    y.toLowerCase // ok: y should have type `String` in this branch
+  }
+}