isolates.md no flutter web support (#7044)

Flutter web no isolates

Just a small notes into the Isolates page about lack of support in
Flutter Web.

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flutter/flutter#179083

---------

Co-authored-by: Amanda Fitch <18406675+antfitch@users.noreply.github.com>

Author: randomizedcoder

src/content/language/isolates.md

@@ -22,6 +22,11 @@ The most common example is in [Flutter][] applications, when you
 need to perform large computations that might otherwise cause the 
 UI to become unresponsive.
 
+:::flutter-note
+Multiple Isolates not supported by Flutter web
+See also: [Concurrency on the web](https://dart.dev/language/concurrency#concurrency-on-the-web)
+:::
+
 There aren't any rules about when you _must_ use isolates, 
 but here are some more situations where they can be useful: