Microsoft MakeCode

Pxt-sample/node_modules/@types/ws/index.d.ts(219,18): error TS2304: Cannot find name 'Set'

I’m working my way through the pxt-sample (commit 575fbd38968751877b3f0ad5622c6cff80f7845d) and I see the following messages about Set.

Have I setup my development system incorrectly?

pxt serve
Using target HDS with build engine none
  target: v2.0.2 /disk1/home/josephwinston/src/Remote/GIT/microsoft/pxt-sample
  pxt-core: v5.6.4 /disk1/home/josephwinston/src/Remote/GIT/microsoft/pxt-sample/node_modules/pxt-core
log strings: 6 files; 0 strings -> sim-strings.json
/disk1/home/josephwinston/src/Remote/GIT/microsoft/pxt-sample/node_modules/@types/ws/index.d.ts(219,18): error TS2304: Cannot find name 'Set'.
building target.json in /disk1/home/josephwinston/src/Remote/GIT/microsoft/pxt-sample...
building /disk1/home/josephwinston/src/Remote/GIT/microsoft/pxt-sample/libs/core
package built; written to binary.hex
building libs/core
building libs/blocksprj
target.json built.
building sim...
[run] cd sim; node ../node_modules/typescript/bin/tsc
../node_modules/@types/ws/index.d.ts(219,18): error TS2304: Cannot find name 'Set'.

***
*** Build failed: target build failed: Exit code: 2 from cd sim; node ../node_modules/typescript/bin/tsc
Error: Exit code: 2 from cd sim; node ../node_modules/typescript/bin/tsc
    at ChildProcess.<anonymous> (/disk1/home/josephwinston/src/Remote/GIT/microsoft/pxt-sample/node_modules/pxt-core/built/nodeutil.js:78:24)
    at ChildProcess.emit (events.js:198:13)
    at maybeClose (internal/child_process.js:982:16)
    at Process.ChildProcess._handle.onexit (internal/child_process.js:259:5)
***

Starting server in /disk1/home/josephwinston/src/Remote/GIT/microsoft/pxt-sample
With pxt core at /disk1/home/josephwinston/src/Remote/GIT/microsoft/pxt-sample/node_modules/pxt-core
starting local ws server at 3233...
---------------------------------------------

To launch the editor, open this URL:
http://localhost:3232/#local_token=c8fb9010-d6f4-4d89-324b-6b89b7103ef7&wsport=3233

---------------------------------------------
opening http://localhost:3232/#local_token=c8fb9010-d6f4-4d89-324b-6b89b7103ef7&wsport=3233