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

lib/6to5/transformation/transformers/es6-classes.js

@@ -102,6 +102,7 @@ Class.prototype.run = function () {
   //
 
   if (superName) {
+    this.closure = true;
     body.push(t.expressionStatement(t.callExpression(file.addDeclaration("inherits"), [className, superName])));
   }
 
@@ -153,7 +154,7 @@ Class.prototype.buildBody = function () {
     }
   }
 
-  if (!this.hasConstructor && superName) {
+  if (!this.hasConstructor && superName && !t.isFalsyExpression(superName)) {
     constructor.body.body.push(util.template("class-super-constructor-call", {
       SUPER_NAME: superName
     }, true));

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 = [];