Support when without expression.

#KT-1666 Fixed

js/js.translator/testFiles/patternMatching/cases/kt1665.kt

@@ -0,0 +1,9 @@
+fun main(args : Array<String>) {
+  val a = 10
+  val b = 3
+  when {
+    a > b -> println("a")
+    b > a  -> println("b")
+    else -> println("Unknown")
+  }
+}

js/js.translator/testFiles/patternMatching/cases/whenWithoutExpression.kt

@@ -0,0 +1,6 @@
+package foo
+
+fun box() = when {
+  1 > 3 -> false
+  else -> true
+}