3.0.0-beta.2 - 2018.05.26

See all changes in #325

Updates based on results of May 2018 TC39 meeting:

• Array.prototype.flatten renamed to Array.prototype.flat
• Symbol.prototype.description moved to stage 3
• Object.fromEntries moved to stage 2
• New Set methods:
  • Moved to stage 2
  • Updated names of methods:
    • Set.prorortyp.except -> Set.prorortyp.difference
    • Set.prorortyp.xor -> Set.prorortyp.symmetricDifference