Port 2aa070be

Original commit message:
    Repair this to match what the runtime correctly does, by first checking if the
    function is a constructor before we access the prototype.

R=mvstanton@chromium.org, joransiu@ca.ibm.com, jyan@ca.ibm.com, michael_dawson@ca.ibm.com
BUG=

Review URL: https://codereview.chromium.org/1811013002

Cr-Commit-Position: refs/heads/master@{#34869}