chore(dtslint): add higher-order observable tests

[cartant]

Description:

I should have included these dtslint tests in #3945 to show that the change does fix the reported issue. It does.

Related issue (if exists): #3841

[cartant]

I don't know what the problem within Travis relates to. I've cleared the cache, but the error persists.

Also, checking the tags for the module mentioned in the error - definitelytyped-header-parser - effects this error:

$ npm dist-tag ls definitelytyped-header-parser
npm WARN notice Due to a recent security incident, all user tokens have been invalidated. Please see https://status.npmjs.org/incidents/dn7c1fgrr7ng for more details. To generate a new token, visit https://www.npmjs.com/settings/~/tokens or run "npm login".
npm ERR! dist-tag ls Couldn't get dist-tag data for definitelytyped-header-parser
npm ERR! code E404
npm ERR! 404 Registry returned 404 for GET on https://registry.npmjs.org/-/package/definitelytyped-header-parser/dist-tags

npm ERR! A complete log of this run can be found in:
npm ERR!     C:\Users\Nicholas\AppData\Roaming\npm-cache\_logs\2018-07-26T23_49_25_070Z-debug.log

[kwonoj]

:wat: error message doesn't...make sense, why we are encountering error for token when try to install something?

[cartant]

@kwonoj Yeah, I know. I don't know whether there is some weirdness happening with npm in general or if the lock file was somehow messed up in f0ac727

However, running npm ci here - using npm v6.2.0 - with the latest master effects the same error on my computer. So I think I'm inclined to do an npm uninstall --save-dev dtslint and npm install --save-dev --save-exact dtslint to update the lock file. Or maybe I should just regenerate the lock file?

[kwonoj]

So I think I'm inclined to do an npm uninstall --save-dev dtslint and npm install --save-dev --save-exact dtslint to update the lock file.

I think that's worth to try. If that still doesn't work, feel free to burn current pkg lock and regenrate. We are relatively free from pkglock since our deps are mostly devdeps, so no worries of prod pkg inconsistency.

[cartant]

An uninstall/install of dtslint fixed it for me on this comuter. See #3959.

[kwonoj]

the mysterious world of npm.

[benlesh]

I'm calling bullshit on npm and merging anyway. haha... Damn the torpedoes!