only set strictMode to true when the useStrict transformer is enabled

lib/6to5/file.js

@@ -316,6 +316,7 @@ File.prototype.parse = function (code) {
   var opts = this.opts;
 
   opts.allowImportExportEverywhere = this.isLoose("es6.modules");
+  opts.strictMode = this.transformers.useStrict.canRun();
 
   return util.parse(opts, code, function (tree) {
     self.transform(tree);

lib/6to5/util.js

@@ -204,7 +204,7 @@ exports.parse = function (opts, code, callback) {
       allowReturnOutsideFunction:  true,
       ecmaVersion:                 opts.experimental ? 7 : 6,
       playground:                  opts.playground,
-      strictMode:                  true,
+      strictMode:                  opts.strictMode,
       onComment:                   comments,
       locations:                   true,
       onToken:                     tokens,