Port 5d68529b

Original commit message:
Scope, like Parser, must be able to operate independent of Isolate and the V8
heap (for background parsing). After the heap-independent phase, there is a heap
dependent phase, during which we do operations such as scope anaylysis.

This CL makes the phases explicit by not telling Scope about the Isolate too
early (during the heap-independent phase, Scope should know nothing about
Isolate). This decreases the probability of accidental code changes which would
add heap-dependent operations into the heap-independent phase.

BUG=

Review URL: https://codereview.chromium.org/911073004

Cr-Commit-Position: refs/heads/master@{#26553}