When used with gulp-source-maps build fails with error "Path must be a string. Received undefined"

After updating my node to "v8.11.1" and npm to "5.6.0" gulp build gets broken if used with "gulp-source-maps"

Here is my gulpfile:
```
"use strict";

let tabReloaderInstance,
gulp = require("gulp"),
gulpSourcemaps = require("gulp-sourcemaps"),
mergeStream = require("merge-stream"),
yargs = require("yargs"),

argv = yargs.argv,
rootPath = argv.JenkinsCartridgesPath || __dirname,

// Babel
babel = require("gulp-babel"),
gulpBrowserify = require("gulp-browserify"),
es2015 = require("babel-preset-es2015");

/* JavaScript paths */
var JS_PATHS = [
{
"wtc" : "int_acf_core/cartridge/js/**/*.js",
"src" : "int_acf_core/cartridge/js/modules_system.js",
"dst" : "int_acf_core/cartridge/static/default/js"
}
];

// generate js
gulp.task("js", () => {
var streams = mergeStream();
var optBrfy = {"debug" : true}

// Needed when task is run from Jenkins so that Browserify can resolve the modules installed with NPM
if (argv.JenkinsBuild) {
optBrfy = xtend(optBrfy, {"paths" : [argv.JenkinsNodeModulesPath]});
}

JS_PATHS.forEach(function(p) {
var srcPath = path.join(rootPath, p.src),
dstPath = path.join(rootPath, p.dst);

streams.add(gulp.src(srcPath)
.pipe(gulpBrowserify(optBrfy))
.pipe(gulpSourcemaps.init())
.pipe(babel({
presets: [es2015]
}))
.pipe(gulpSourcemaps.write('./'))
.pipe(gulp.dest(dstPath)));
});

return streams;
});
```

js task fails with error "Path must be a string. Received undefined"

I traced the error to "node_modules/gulp-babel/index.js"
line 39

where you gulp-babel has following code as this point:
```
if (file.sourceMap && res.map) {
res.map.file = replaceExtension(res.map.file);
applySourceMap(file, res.map);
}
```

Problem is that res.map.file here is undefined even though file.sourceMap and res.map has big JSON objects returned.

this breaks the build.
Before update this setup was working properly.

Start with the gulpfile's js task and reproduce the failure using gulp-browserify, gulp-sourcemaps, gulp-babel, Node v8.11.1, and npm 5.6.0. Inspect node_modules/gulp-babel/index.js at line 39 and trace why res.map.file is undefined when the source map is applied. Done means the js task builds successfully with source maps enabled.

Written by the indexing model from the issue text.