Typescript 2.6.1 Compilation Errors

[thejohnhoffer]

If I checkout master today into an empty directory and run npm i from npm v5.0.3 for node v8.0.0, I've installed all the packages in the this package-lock.json, which includes typescript v2.6.1.

When I try to run npm run dev-server as usual, typescript highlights an unused variable:

    ERROR in ./src/neuroglancer/chunk_manager/backend.ts
    [tsl] ERROR in /Users/John/2017/fall/neuroglancer/src/neuroglancer/chunk_manager/backend.ts(586,9)
          TS6133: 'visibleChunksChanged' is declared but its value is never read.

as well as a couple technical edge cases:

    ERROR in ./src/neuroglancer/util/google_oauth2.ts
    [tsl] ERROR in /Users/John/2017/fall/neuroglancer/src/neuroglancer/util/google_oauth2.ts(254,9)
          TS2531: Object is possibly 'null'.

    ERROR in ./src/neuroglancer/webgl/shader.ts
    [tsl] ERROR in /Users/John/2017/fall/neuroglancer/src/neuroglancer/webgl/shader.ts(109,9)
          TS2531: Object is possibly 'null'.

    ERROR in ./src/neuroglancer/chunk_manager/frontend.ts
    [tsl] ERROR in /Users/John/2017/fall/neuroglancer/src/neuroglancer/chunk_manager/frontend.ts(222,5)
          TS2322: Type 'RefCounted' is not assignable to type 'T'.

A quick fix

Anyway, I just took the simplest possible steps to fix these errors in this pull request.

A side note

Before or after the changes, here is a potentially relevant warning that I ignore on every build:

ts-loader: Using /Users/John/2017/fall/neuroglancer/config/typescript_compiler_shim.js@2.6.1. This version may or may not be compatible with ts-loader.