When (user) assertions are enabled in dart2js, we automatically update
the environment as if -Ddart.web.assertions_enabled=true were
passed. This allows dev-only code to be gated behind
`const bool.fromEnvironment('dart.web.assertions_enabled')` without
having to supply a redundant option manually.
Change-Id: I40d670b6fe87d5e27b2b8d50599261cca8a79a42
Bug: b/254543452
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/270301
Reviewed-by: Sigmund Cherem <sigmund@google.com>
Mayank Patke