[WIP] Feature/micromap

[rkirov]

Rebased on top of #1208 (original micro map) and #1178 (view perf improvements).

tree benchmark (with dart2js):
pr 1178 (baseline) - 470ms +/- 12%, 42.6 Mb heap
this PR - 450ms +/- 12%, 41.8 Mb heap

The confidence intervals are quite large so I am not sure if this is indeed better or we are looking into noise. Also, running the benchmark for extended period seems to affect the numbers significantly (as some point I saw the baseline drop to 430ms itself).

More work needs to be done on the iterators (they should be views into the micromaps, instead of holding their own values).

(profile-baseline)

(profile-pr)

(baseline heap)

(micromap heap)

[rkirov]

Example of differences between MicroMap and HashMap due to Iterables:

  var map = new *Map<num, num>();                                             
  var it = map.keys.iterator;                                                    
  map[1] = 1;                                                                    
  it.moveNext(); 

MicroMap - fine
HashMap - throws 'modification during iteration'

    var map = new *Map<num, num>();
    map[1] = 1;
    var it = map.values.iterator;
    map[1] = 2;
    it.moveNext();
    print(it.current);

MicroMap - 1
HashMap - 2

[mhevery]

@rkirov Can you squash all of the SHAs into a single SHA.

[jbdeboer]

We could not find a performance improvement here. Closing