Without this check, instantiation of the wasm module would fail on
DefineOwnProperty on the exports object for the duplicate export.
Now we detect this as validation error.

R=rossberg@chromium.org, titzer@chromium.org, ahaas@chromium.org

Review-Url: https://codereview.chromium.org/2065043002
Cr-Commit-Position: refs/heads/master@{#37038}