Removes API migration from the description and ensures the disclaimer appears when run in a help context.
Sample output:
```
$ dart bin/dartdev.dart fix
Fix Dart source code.
This tool looks for and fixes analysis issues that have associated automated fixes.
To use the tool, run either 'dart fix --dry-run' for a preview of the proposed changes for a project, or
'dart fix --apply' to apply the changes.
Note: The `fix` command is provisional and subject to change or removal in future releases. Feedback is
welcome!
Usage: dart fix [arguments]
-h, --help Print this usage information.
-n, --dry-run Preview the proposed changes but make no changes.
--apply Apply the proposed changes.
Run "dart help" to see global options.
$ dart bin/dartdev.dart fix -n
Note: The `fix` command is provisional and subject to change or removal in future releases. Feedback is welcome!
Computing fixes in dartdev (dry run)... 15.2s
Nothing to fix!
$ dart bin/dartdev.dart fix --help
Fix Dart source code.
This tool looks for and fixes analysis issues that have associated automated fixes.
To use the tool, run either 'dart fix --dry-run' for a preview of the proposed changes for a project, or
'dart fix --apply' to apply the changes.
Note: The `fix` command is provisional and subject to change or removal in future releases. Feedback is
welcome!
Usage: dart fix [arguments]
-h, --help Print this usage information.
-n, --dry-run Preview the proposed changes but make no changes.
--apply Apply the proposed changes.
Run "dart help" to see global options.
```
Change-Id: I2fc03651d8a1605a9729bb4d62e382f533509d99
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/175343
Commit-Queue: Phil Quitslund <pquitslund@google.com>
Reviewed-by: Brian Wilkerson <brianwilkerson@google.com>
Reviewed-by: Devon Carew <devoncarew@google.com>
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