Drop TODO to remove isHostChecked option

See #37318

This enables asserts in the dart2js compiler and is worth keeping.
Update the comment to better express the behavior and remove the TODO to
drop it.

Change-Id: I2e1eef74eac7949812670dd1c6294641ce80a4a8
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/106940
Auto-Submit: Nate Bosch <nbosch@google.com>
Reviewed-by: Bob Nystrom <rnystrom@google.com>
Commit-Queue: Bob Nystrom <rnystrom@google.com>

Author: natebosch

pkg/smith/lib/configuration.dart

@@ -303,7 +303,7 @@ class Configuration {
 
   final bool isCsp;
 
-  // TODO(rnystrom): Remove this when Dart 1.0 is no longer supported.
+  /// Enables asserts in the dart2js compiler.
   final bool isHostChecked;
 
   final bool isMinified;