[dart2js] Identify required powerset bits

[fishythefish]

The current scaffolding for the powerset domain defines a few bits:

• true/false/null/other
• interceptor/not interceptor/null interceptor

We may need to confirm that these bits are still useful and in a usable state. We also need to determine what other bits the powerset domain should support and sort them by priority.

[rakudrama]

For each optimization, the value space can be partitioned into values that allow an optimization, and some sister bits that cover the cases where the optimization does not apply.

We have two hierarchies of 'fake' types so that the type-masks can be used to make distinctions within a natural type. These are will be better expressed as powerset subdomains.

JSArray has fake subclasses to model the varieties of list - each subclass permits some optimization - e.g. fixed-length lists have a .length that can be hoisted.
The powerset domain would include other JSIndexables, including some @Native classes like NodeList.

JSInt has a similar set of partioning subclasses.
JSUInt31 targets bit-operation optimizations.
In addition to small non-negative integers, it would be useful to model -0.0 since the absence of -0.0 from otherwise non-negative values allows lowering to %.

I would take on the JSInt-subclass-replacement domain later than some others since the transfer functions are harder to write.