Use Object.assign instead of merge, mergeInto, mixInto and copyProperties

This makes it easier to contribute without having to learn a bunch of
slightly different helpers and remember their slightly different
signatures and semantics.

We'll probably start using ES7 spread properties instead of merge in the
future when at least one more transpiler supports it.
Merge pull request #2303 from sebmarkbage/dropmerge

Use Object.assign instead of merge, mergeInto, mixInto and copyPropertie...