[Web] getExistingAnonymousId does not decode JSON-stringified ajs_anonymous_id from localStorage

[Jharrison23]

Bug

When analytics.js (the Segment JavaScript SDK) writes ajs_anonymous_id to localStorage, it uses JSON.stringify(), which wraps the UUID in surrounding double-quote characters. The Flutter SDK's getExistingAnonymousId() in packages/core/lib/utils/store/web.dart reads this value via localStorage.getItem() but does not decode it, so the quotes become part of the anonymous ID string.

This means any Flutter web app running on a domain where analytics.js has previously set ajs_anonymous_id will pick up a corrupted anonymous ID (e.g. "d039d945-..." instead of d039d945-...), causing identity mismatches between the JS and Flutter SDKs.

Expected behavior

anonymousId should be a clean UUID:

d039d945-572c-4148-b072-2b54ef52102a

Actual behavior

anonymousId includes surrounding quotes, which get double-escaped when serialized to analytics-flutter-userInfo.json:

{"anonymousId":"\"d039d945-572c-4148-b072-2b54ef52102a\""}

Proposed fix

json.decode the raw localStorage value in getExistingAnonymousId() before returning it, with a try/catch fallback for values that are already plain strings.