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Error when compiling a language that requires another language

See original GitHub issue

How to reproduce Create a language testa.js

/*
Language: testa
Requires: testb
*/

export default function(hljs) {
  return {
    name: 'testa'
  };
}

that requires a language testb.js

/*
Language: testb
*/

export default function(hljs) {
  return {
    name: 'testb'
  };
}

Then compile testa

node .\tools\build.js -n testa

You will receive the following messages on the terminal:

Starting build.
Finished build.
... testa requires testb
(node:3900) UnhandledPromiseRejectionWarning: TypeError: Cannot read property 'requires' of undefined
    at filter (D:\Projekte\highlight.js_parser\highlight.js\tools\lib\dependencies.js:74:10)
    at Object.buildBrowser [as build] (D:\Projekte\highlight.js_parser\highlight.js\tools\build_browser.js:25:15)
    at async doTarget (D:\Projekte\highlight.js_parser\highlight.js\tools\build.js:87:3)
(node:3900) UnhandledPromiseRejectionWarning: Unhandled promise rejection. This error originated either by throwing inside of an async function without a catch block, or by rejecting a promise which was not handled with .catch(). To terminate the node process on unhandled promise rejection, use the CLI flag `--unhandled-rejections=strict` (see https://nodejs.org/api/cli.html#cli_unhandled_rejections_mode). (rejection id: 1)
(node:3900) [DEP0018] DeprecationWarning: Unhandled promise rejections are deprecated. In the future, promise rejections that are not handled will terminate the Node.js process with a non-zero exit code.

Trying to use the languages in tools/developer.html shows that the compilation indeed did not succeed.