Unable to concatenate file with multiple sourcemaps

[mattdsteele]

I've built a UMD bundle with Webpack 3.0.0, using devtool: '#source-map' as my source map generation option. It ends up producing a build with multiple //# sourceMappingUrl pragmas. For example:

/******/ ([
/* 0 */
/***/ (function(module, exports) {

module.exports = __WEBPACK_EXTERNAL_MODULE_0__;

/***/ }),
/* 1 */
/***/ (function(module, exports, __webpack_require__) {
 /// code
};
//# sourceMappingURL=common.js.map

/***/ }),
/* 2 */
/***/ (function(module, exports, __webpack_require__) {

"use strict";
 /// code
};
//# sourceMappingURL=predicates.js.map

/// etc, to end of file:
//# sourceMappingURL=build.js.map

Only the final build.js.map exists on disk. But this code finds the first sourceMappingUrl and attempts to read it, which results in errors such as:

Warning: Unable to read "C:\code\node_modules\sample\common.js.map" file (Error code: ENOENT). Use --force to continue.

I'm not familiar enough with the source map spec to know how to resolve this, but other tools (gulp, Chrome) seem to read this file fine.