[ts] enable support for iterators in browsers

[spalger]

I was surprised when I tried to spread a Set in TypeScript and the browser complained about Set.slice() not being defined. This is because TypeScript does not automatically enable support for iterators when targeting earlier ES versions, like we do in the browser, unless you use the "downlevelIteration": true compiler option. This injects some helpers into the necessary files for reading/spreading iterators, which can be stuffed behind an import statement with using the "importHelpers": true compiler option and include tslib in our dependencies. This is already a dependency of several of our packages, so it shouldn't cause any additional modules.

[spalger]

00:04:27.831 ERROR: 'yarn kbn bootstrap' caused changes to the following files:
00:04:27.832
00:04:27.832 packages/kbn-es/yarn.lock
00:04:27.832 packages/kbn-i18n/yarn.lock
00:04:27.832 packages/kbn-test/yarn.lock

Weird, when I ran yarn kbn bootstrap locally and those packages reported "Already up-to-date", yarn.lock file wasn't updated until I deleted the .yarn-integrity files... ugh

[spalger]

00:14:19.256     Timeout - Async callback was not invoked within the 5000ms timeout specified by jest.setTimeout.
00:14:19.256 
00:14:19.267       37 |   const buildFiles = PLUGIN.buildSourcePatterns;
00:14:19.276       38 | 
00:14:19.282     > 39 |   it('removes development properties from package.json', async () => {
00:14:19.285          |   ^
00:14:19.286       40 |     expect(PLUGIN.pkg.scripts).not.toBeUndefined();
00:14:19.289       41 |     expect(PLUGIN.pkg.devDependencies).not.toBeUndefined();
00:14:19.291       42 | 

Similar tests in other PRs have failed so I created #23005 to move these tests to the jest integration suite.

[elasticmachine]

💚 Build Succeeded

• continuous-integration/kibana-ci/pull-request

[tylersmalley]

Is this supposed to be a devDependency? We only use the typescript runtime during development, correct?

[spalger]

Nope, TypeScript will inject require statements for tslib when we use things like a for...of over an iterable, so tslib needs to be installed outside of dev too.

[tylersmalley]

Ah, gotcha. Thanks.

[spalger]

6.5/6.x: 489779a
6.4.2: 73c8166