Incremental transformations required to use transformations with Expression Compilation

[sjindel-google]

Although asynchronous expressions are disallowed by the expression compilation API, asynchronous
code can still appear within a synchronous expression:

new Future.value(0).then((x) async => x + 1);

This expression can be used within a sync function; however, to be run in the VM, it currently requires
the async. transformation. Unfortunately, we don't have an API for transformations to be run incrementally yet. In the short term, we'll have to lock out expressions containing any asynchronous code from the VM's debugger.