remove runtime option

lib/6to5/file.js

@@ -60,7 +60,6 @@ File.normaliseOptions = function (opts) {
     comments:               true,
     filename:               "unknown",
     modules:                "common",
-    runtime:                false,
     loose:                  [],
     code:                   true,
     ast:                    true
@@ -95,10 +94,6 @@ File.normaliseOptions = function (opts) {
     sourceMapName:  opts.filenameRelative
   });
 
-  if (opts.runtime === true) {
-    opts.runtime = "to5Runtime";
-  }
-
   if (opts.playground) {
     opts.experimental = true;
   }
@@ -207,10 +202,8 @@ File.prototype.addHelper = function (name) {
   if (declar) return declar.id;
 
   var ref;
-  var runtimeNamespace = this.opts.runtime;
-  if (runtimeNamespace) {
-    name = t.identifier(t.toIdentifier(name));
-    return t.memberExpression(t.identifier(runtimeNamespace), name);
+  if (this._runtime) {
+    return t.memberExpression(this._runtime, name);
   } else {
     ref = util.template(name);
   }