[Deleted user (ghost)]

Similar to #11354, but simpler.

• Keeps using Map<T> instead of Map<K, V>, with K assumed to be string. For number-keyed maps we now use sparse arrays.
• Does not include Set -- this can be done later.
• Does not include sortInV8ObjectInsertionOrder; changes baselines instead. We won't be able to pass all tests when using the shim map, but we no longer run tests on node 0.10 anyway.

Added functions

Name	Use
arrayFrom	Converts anIterator to an Array. arrayFrom(map.keys()) replaces Object.keys(map). arrayFrom(map.values()) also lets us remove reduceProperties, which was only used to compute that.
forEachEntry, forEachKey	These replace for-in loops with early termination, forEachProperty, and someProperties.
copyEntries	Replaces copyProperties.
createMultiMap	Replaces multiMapAdd and multiMapRemove.
mapsAreEqual	Replaces a use of equalOwnProperties. Used only in reuseProgramStructure.ts, so moved it there.
multiMapSparseArrayAdd	Replaces a use of multiMapAdd on what is now a sparse array. Used only in factory.ts, so moved it there.
mapEntries	Replaces mapObject.

We also get rid of isEmpty and replace with map.size === 0.