perf(bracket): Optimize calling methods on objects.

Optimize the closures used for calling methods on objects by getting rid
of an intermediate "field access" that loads the function from the object.
This allows us to use InstanceMirror.invoke more directly.

Performance
===========
The performance implications of this change look good. On the native Dart VM,
calling methods through the reflective path becomes 4.5x faster. When compiled
to JavaScript, the difference is even bigger and it is 6.7x faster.

Dart VM:
  new: o.f() => g: 18,885,385.384 ops/sec  r: 2,252,149.057 ops/sec   h: 2,233,514.96 ops/sec   i: 32,662,458.941 ops/sec
  old: o.f() => g: 18,118,349.692 ops/sec  r:   496,119.238 ops/sec   h: 6,234,893.68 ops/sec   i: 31,472,943.499 ops/sec

Dart2js:
  new: o.f() => g:  2,618,328.337 ops/sec  r: 1,234,723.632 ops/sec   h: 1,234,745.10 ops/sec   i:  7,507,441.131 ops/sec
  old: o.f() => g:  2,427,133.253 ops/sec  r:   184,727.903 ops/sec   h:   500,226.70 ops/sec   i:  7,461,638.467 ops/sec