ad hoc fix for es5build.js

es5build.js.diff

diff --git a/tools/build/es5build.js b/tools/build/es5build.js
index b16f9d9..25b5ae6 100755
--- a/tools/build/es5build.js
+++ b/tools/build/es5build.js
@@ -56,7 +56,10 @@ function run(config) {
     .pipe(sourcemaps.init())
     .pipe(through2.obj(function(file, encoding, done) {
       var self = this;
-      fs.readFile(file.path.replace('.js', '.map'), function(error, buffer) {
+      var filePath = file.path.replace('.js', '.map');
+      var exists = fs.existsSync(filePath);
+      filePath = exists ? filePath : file.path.replace('.js', '.es6.map');
+      fs.readFile(filePath, function(error, buffer) {
         if (error) {
           return done(error);
         }