allow vm args to be specified

[Andersmholmgren]

I'm using async and enums in my test code and need to be able to specify flags to dart to enable them

[nicolasgarnier]

You're right that we need to have this capability. One of the issue is that we are currently running VM tests with pub run and it's not possible to pass flags to the underlying VM using pub run but we are planning on moving away from pub run and using dart directly instead. We loose automatic transformers but get rid of a bunch of issues with pub run (forced checked mode, no ability to gather code coverage data, Platform.script.path not working etc...).