Arrow function parameter lists are parsed as expressions. When an identifier
is found a VariableProxy is created and added to the list of unresolved
variables for the scope. When parsing a function lazily, the scope has been
already resolved, so with this patch only the VariableProxy is created,
without adding it as an unresolved variable in the scope.

BUG=v8:3501
LOG=Y

Review URL: https://codereview.chromium.org/880253004

Cr-Commit-Position: refs/heads/master@{#26328}