Build: support passing VM flags to subprocesses

[sigmundch]

Users would like to be able to plumb VM flags to subprocesses. The main motivating example at this time is to specify a different --old-gen-heap-size to the VM when invoking dart2js to prevent OOM errors. See https://github.com/dart-lang/sdk/issues/27883 for an example

@jakemac53 - not sure if this is the right repo to track this?

[grouma]

@jakemac53 should this be bounced over to the build package?

[robbecker-wf]

and possibly might need a similar issue for webdev

[jakemac53]

It should be done with an environment variable, so only build will have to know about it.

[jakemac53]

How do we want to support passing multiple arguments here? separate by commas or...?

[jakemac53]

Actually I will just go with space separated args

[jakemac53]

planned name for the environment variable is BUILD_DART2JS_VM_ARGS

[jakemac53]

This has been publish in build_modules 2.7.0. Multiple arguments can be provided by separating them with spaces.

[robbecker-wf]

Thanks! Can confirm this does in fact get --old-gen-heap-size through to dart2js when running webdev.