Fixes code execution.
Fixes code execution.

If we allow multiple postMessage calls to the same worker in the future,
this would've led to overwritten code.

colemakdvorak/parallel.js,SRobertZ/parallel.js,blackrabbit99/doubleW,adambom/parallel.js,colemakdvorak/parallel.js,kidaa/parallel.js,parallel-js/parallel.js,adambom/parallel.js,1aurabrown/parallel.js,blackrabbit99/doubleW,parallel-js/parallel.js,blackrabbit99/parallel.js,1aurabrown/parallel.js,SRobertZ/parallel.js,blackrabbit99/parallel.js,kidaa/parallel.js