[api-extractor]: Upgrade typescript minor to 3.4

[rudolf]

Upgrades the typescript version used by api-extractor to analyze projects.

[octogonz]

FYI you are upgrading the compiler that is used to compile API Extractor itself. If you want to upgrade the compiler that API Extractor uses when analyzing a project, that's this line:

https://github.com/Microsoft/web-build-tools/blob/5fcdc42baf2dab3a03f7608c6d7673e08eb4b2fd/apps/api-extractor/package.json#L41

[rudolf]

@octogonz Thanks, I've changed the PR to correctly upgrade typescript.

[rudolf]

I'm really battling with the rush build system here. @octogonz or @iclanton Can you offer some guidance for how I should go about upgrading api-extractor's typescript version?

[octogonz]

@iclanton maybe you could help with this? It seems that API-Extractor-on-TypeScript 3.3.3333 is not able to analyze projects that build using rush-stack-compiler-3.2, whereas API-Extractor-on-TypeScript 3.1.6 (our current master) seems to work okay.

Here's an example error when building the rushell library (which uses rush-stack-compiler-3.2):

Warning - [api-extractor] Warning: wbt/common/temp/node_modules/.registry.npmjs.org/typescript/3.2.4/node_modules/typescript/lib/lib.es2015.iterable.d.ts:43:6 - (TS2585) 'Symbol' only refers to a type, but is being used as a value here. Do you need to change your target library? Try changing the lib compiler option to es2015 or later.

@iclanton I wonder if RSC should always invoke API Extractor using the --typescript-compiler-folder option.

We could probably "solve" this by upgrading all the WBT projects to the latest RSC, but the scenario is supposed to work: The whole idea is that we don't want to have to maintain separate API Extractor release branches for each TypeScript compiler release, and we also don't want to be forced to constantly upgrade everything to the latest compiler.

[octogonz]

@iclanton I wonder if RSC should always invoke API Extractor using the --typescript-compiler-folder option.

Actually, it appears that we're already doing that:

ApiExtractorRunner.ts

          message.handled = true;
        },
        typescriptCompilerFolder: ToolPaths.typescriptPackagePath
      };

I'm going to update this PR and see if I can figure out what's going on.

[octogonz]

Weird... I started over with a clean upgrade: API Extractor now uses typescript 3.4 for analysis, while API Extractor itself is still built using RSC 3.2. It worked perfectly the first time. The typescriptCompilerFolder feature seems to be working perfectly. I'm not sure what we were doing wrong before, but... problem solved, so I'm not going to investigate it heheh.

[rudolf]

Awesome 🎉

[octogonz]

This feature was published with the API Extractor beta release 7.0.41