As of Dart SDK 1.22.0-dev.5.0, `Process.exitCode` is no longer
mutable (that SDK version picks up https://github.com/dart-lang/sdk/commit/e5a16b1ca56414af3008afb8833660f95a9d06da).
This change allows the tools code to pass analysis in sdk versions both
before and after that change, to allow for analysis against both the host and
target sdks.