Block imports to unsupported libraries in web tests

[natebosch]

We disallow imports to dart:io in our production toolchains: webdev, bazel. We currently allow imports in flutter, and when running web tests, with runtime failures instead. To avoid surprises - both differences in behavior and for earlier detection of problems, we should also disallow the imports in web tests.

We don't have any easy way to detect this. Ideally dart2js would offer a way to fail for these imports.