Pass filename to ast-transform for sourcemaps (#140)

This is a solution for #139. Once I had it reproduced properly, it was relatively clear what was going 😄

lib/browserify-transform.js

@@ -35,11 +35,15 @@ module.exports = function yoYoify (file, opts) {
   }
   function end (cb) {
     var src = Buffer.concat(bufs).toString('utf8')
+    var flags = (opts && opts._flags) || {}
+    var basedir = flags.basedir || process.cwd()
+    var filename = path.relative(basedir, file)
     var res
     try {
-      res = transformAst(src, { ecmaVersion: 8, parser: acorn }, walk)
-      if (opts && opts._flags && opts._flags.debug) {
-        res = res.toString() + '\n' + convertSourceMap.fromObject(res.map).toComment() + '\n'
+      res = transformAst(src, { ecmaVersion: 8, parser: acorn, inputFilename: filename }, walk)
+      if (flags.debug) {
+        var sm = convertSourceMap.fromObject(res.map).toComment()
+        res = res.toString() + '\n' + sm + '\n'
       } else {
         res = res.toString()
       }