Remove unnecessary ! operator.

[asashour]

These cause failure on the SDK build, with a log of:

warning - test_benchmarks/benchmarks/scroll.dart:88:66 - The '!' will have no effect because the receiver can't be null. Try removing the '!' operator. - unnecessary_non_null_assertion
warning - test_benchmarks/benchmarks/scroll.dart:98:34 - The '!' will have no effect because the receiver can't be null. Try removing the '!' operator. - unnecessary_non_null_assertion
warning - test_benchmarks/benchmarks/scroll.dart:104:21 - The '!' will have no effect because the receiver can't be null. Try removing the '!' operator. - unnecessary_non_null_assertion

Pre-launch Checklist

• I read the Contributor Guide and followed the process outlined there for submitting PRs.
• I read the Tree Hygiene wiki page, which explains my responsibilities.
• I read the Flutter Style Guide recently, and have followed its advice.
• I signed the CLA.
• I listed at least one issue that this PR fixes in the description above.
• I updated/added relevant documentation (doc comments with ///).
• I added new tests to check the change I am making, or this PR is test-exempt.
• All existing and new tests are passing.

If you need help, consider asking for advice on the #hackers-new channel on Discord.

[bwilkerson]

Note: These diagnostics are breaking the Dart SDK's flutter-analyze bot, which we depend on to tell us when changes to the analyzer will break the roll of the Dart SDK into flutter.