During parsing, we now keep track of the first spread seen in an array
literal (if any), and make use of that information when creating the
FixedArray backing store representing the constant elements for array
literal materialization.
The old code tried to do this by setting the generated JSArray's length
in ArrayLiteral::BuildConstantElements(), but that Array length is never
read by the rest of the literal materialization code (it always uses
the length of the FixedArray backing store).
BUG=v8:4298
LOG=n
Review URL: https://codereview.chromium.org/1225223004
Cr-Commit-Position: refs/heads/master@{#29684}
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