Remove dart:isolate changes from 1.13 changelog, documented spawnUri environment argument
Remove dart:isolate changes from 1.13 changelog, documented spawnUri environment argument

Closes https://github.com/dart-lang/sdk/issues/24757
helps https://github.com/dart-lang/sdk/issues/24474

R=sethladd@google.com

Review URL: https://codereview.chromium.org/1423323003 .

dart-archive/dart-sdk,dartino/dart-sdk,dart-archive/dart-sdk,dartino/dart-sdk,dartino/dart-sdk,dart-lang/sdk,dartino/dart-sdk,dartino/dart-sdk,dart-lang/sdk,dartino/dart-sdk,dart-lang/sdk,dartino/dart-sdk,dart-lang/sdk,dart-archive/dart-sdk,dartino/dart-sdk,dart-archive/dart-sdk,dart-archive/dart-sdk,dart-archive/dart-sdk,dart-archive/dart-sdk,dart-lang/sdk,dart-lang/sdk,dart-archive/dart-sdk,dart-archive/dart-sdk,dartino/dart-sdk,dart-lang/sdk,dart-lang/sdk