[dart2js] Minor strong mode fix in inference.
[dart2js] Minor strong mode fix in inference.

Change-Id: I116ac7b5616735abdbc36548d3b2ff53d2307e5f
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/144983
Commit-Queue: Joshua Litt <9f8daed1ab51574c9a48ffacb03e4a1adb4f880c@google.com>
Reviewed-by: Mayank Patke <e0355b78204b67a55a952941c68c8f1b40235938@google.com>

dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk