add isFalsyExpression check to superName and make classes a closure if they're inheriting

lib/6to5/types/index.js

@@ -74,6 +74,17 @@ addAssert("Expression", t.isExpression);
 
 //
 
+t.isFalsyExpression = function (node) {
+  if (t.isLiteral(node)) {
+    return !node.value;
+  } else if (t.isIdentifier(node)) {
+    return node.name === "undefined";
+  }
+  return false;
+};
+
+//
+
 t.toSequenceExpression = function (nodes, scope) {
   var exprs = [];