Fix Dart 2 runtime issue on DDC for the web
Fix Dart 2 runtime issue on DDC for the web

`CommandRunner.run` will return a `Future<T>` and in this case the `T`
is `dynamic`.

Change-Id: Ief156cfc298d3747da805c26e81d34ed7628d00b
Reviewed-on: https://dart-review.googlesource.com/66520
Reviewed-by: Jenny Messerly <6285f43aa2dd0931180012d62b847adf2c37a418@google.com>
Commit-Queue: Nate Bosch <dc212c74d8afde9c690fdd09e74d3e5c5d5a3de0@google.com>

dartino/dart-sdk,dart-lang/sdk,dart-lang/sdk,dartino/dart-sdk,dart-archive/dart-sdk,dart-lang/sdk,dart-archive/dart-sdk,dartino/dart-sdk,dartino/dart-sdk,dartino/dart-sdk,dart-archive/dart-sdk,dartino/dart-sdk,dart-archive/dart-sdk,dartino/dart-sdk,dart-lang/sdk,dart-lang/sdk,dartino/dart-sdk,dart-lang/sdk,dart-lang/sdk,dart-archive/dart-sdk,dart-archive/dart-sdk,dartino/dart-sdk,dart-archive/dart-sdk,dart-archive/dart-sdk,dart-lang/sdk,dart-archive/dart-sdk