Fix running browser tests that use deferred loading

[jakemac53]

Fixes #2088

Pass --disable-program-split so that no splitting actually happens.

I first tried fixing this more "properly" by serving all the output files but this was a lot more complex and supporting stack trace mapping in particular was looking very challenging.

[gmpassos]

This needs to be documented somewhere:

When a test is compiled to JS, the parameter --disable-program-split is passed to dart2js, preventing it from splitting the program to avoid issues when serving the split parts in browser tests. Deferred libraries are the ones that require the compiler to split the program, and they shouldn't be affected by --disable-program-split.