wat2wasm: data section memory index

Take the following code:

(module
  (memory $n 1)
  (memory $m 1)
  (data (memory $m) (i32.const 0))
)

Compile.

The data section has bytes:

; data segment header 0
0000012: 02                                        ; segment flags
0000013: 41                                        ; i32.const
0000014: 00                                        ; i32 literal
0000015: 0b                                        ; end

As you can see from spec, flag == 2 expects memory index u32, but wabt omits it.

If you try to compile that code in JS, you'll see the error:

CompileError: WebAssembly.Module(): invalid memory index 65 for data section (having 2 memories) @+18

Adding memory index after segment flag solves the issue (watr has that fixed).

Start by reproducing the example with the wat2wasm entry point and inspect the emitted data section bytes for the segment using memory $m. Compare the encoding with the WebAssembly data-section specification and validate the result by compiling the output in JavaScript; done means the memory index is emitted and the module compiles successfully.