[es6] Fix WeakMap/Set built-ins subclasssing (issues 3101, 3330).

Reland "[es6] Fix Function and GeneratorFunction built-ins subclassing." (issues 3101, 3330).

Implement flag and source getters on RegExp.prototype (issues 3715, 4528).

Performance and stability improvements on all platforms.

Cr-Commit-Position: refs/heads/4.8.213@{#1}
Cr-Branched-From: 208744bc1080916b437d17ecb550c2ff75d9d975-refs/heads/master@{#31759}