Tests fail when upgrading dependencies

[apisim]

To reproduce:

$ git clone https://github.com/redhat-developer/yaml-language-server.git
...

$ cd yaml-language-server/

$ rm package-lock.json

$ rm yarn.lock

$ yarn install
...

$ yarn compile
yarn run v1.22.4
$ installServerIntoExtension ./out ./package.json ./tsconfig.json && tsc -p .
Copying package.json to extension's server location...
Updating server npm modules into extension's server location...

$ yarn test

(a slew of errors - 139 to be exact - that are _not_ the occasional timeout errors)

The important part is to remove/delete yarn.lock when using yarn instead of npm.

Similarly, set in package.json the version for "vscode-json-languageservice" to be "^3.5.2". Then run yarn install (or just yarn) followed by yarn compile and yarn test.

The errors fall loosely into six categories. Some of the fixes are needed due to breaking changes in "vscode-json-languageservice" since v3.3.0 (the version "yaml-language-server" seems to be using currently). For example, a function used to return in some cases null now returns undefined instead.

[JPinkney]

Can you submit a PR to fix this?

[apisim]

Created PR #252.