The parser can fold together comma separated declarations
into a single block. The AsmWasmBuilder needs to support
this case in order to handle asm.js modules that use this form.

BUG= https://code.google.com/p/v8/issues/detail?id=4203
TEST=mjsunit/asm-wasm
R=aseemgarg@chromium.org,titzer@chromium.org
LOG=N

Review URL: https://codereview.chromium.org/1697423003

Cr-Commit-Position: refs/heads/master@{#34048}