// Really trivial malloc() implementation. We just allocate bytes sequentially from the start of
  // the heap, and reset the whole heap to empty at the start of each request.
  extern byte __heap_base;   // Start of heap -- symbol provided by compiler.

  byte* heap = NULL;         // Current heap position.
  void* last_malloc = NULL;  // Last value returned by malloc(), for trivial optimizations.

  void* malloc(size_t n) {
    last_malloc = heap;
    heap += n;
    return last_malloc;
  }

Am I missing something? How can this work? __heap_base is never used, so first malloc returns a pointer to (void * )NULL;

Shouldn't it be:

  byte *heap = &__heap_base;

or something to that effect?

I haven't read the rest of the code, nor know how WebAssembly works, but I can't make sense of that snippet.