Use native maps where possible

[ghost]

Note: This code needs a lot of cleanup; posted this as soon as I could get good performance numbers. I'll refactor it later if it looks like we will go ahead with this. Tests pass; travis is failing in the linter.

Memory performance numbers are lot more stable than time, and show a consistent 5% benefit.
The code without maps uses the same amount of memory, because we do not use a shim: we use a dictionary-mode object directly.

It works like this:

export const createMap: <T>() => Map<T> = realMaps
    ? <T>() => new Map<T>()
    : createDictionaryModeObject;

With similar conditions on every map-using function. e.g.:

//will rename this later.
export const _g: <T>(map: Map<T>, key: string) => T = realMaps
    ? <T>(map: NativeMap<T>, key: string) => map.get(key)
    : <T>(map: MapLike<T>, key: string) => map[key];

Map is an interface containing only a brand, so it can't be used directly, only through these functions. It's like an abstract data type.

Performance

Tested with the vscode source code.

Current

(tsc -p src --noEmit --diagnostics where tsc --version is 2.1.0-dev.20160928)

Measured	Average	Samples
Memory used	721556	723926, 717774, 722967
Parse time	7.18	6.84, 7.25, 7.44
Bind time	2.38	2.02, 2.59, 2.52
Check time	12.16	10.52, 11.96, 14.00
Total time	21.71	19.37, 21.81, 23.95

I use --noEmit because the emit phase is dominated by I/O write time.

Native maps

Measured	Average	Relative	Samples
Memory used	683115	95%	677958, 684812, 686574
Parse time	6.98	97%	6.79, 7.23, 6.92
Bind time	1.97	83%	1.67, 2.16, 2.08
Check time	11.11	91%	10.60, 11.65, 11.09
Total time	20.06	92%	19.06, 21.03, 20.09

New code without native maps

(Latest version of node, running this branch with the code changed to disable using native maps.)

Measured	Average	Relative	Samples
Memory used	725144	100%	716993, 728782, 729658
Parse time	7.66	107%	7.58, 6.83, 8.57
Bind time	2.14	90%	2.03, 2.13, 2.27
Check time	11.69	96%	11.63, 11.45, 11.99
Total time	21.49	99%	21.23, 20.41, 22.83

[ghost]

Closing in favor of #11354, which uses maps directly (using infix map.get(key) syntax) rather than through an abstract data type.