`-sWASM_ESM_INTEGRATION` incompatibility with metadce

[guybedford]

When building with -sWASM_ESM_INTEGRATION and say -O3, there's a JS optimizer assertion failure.

Example:

emcc test/hello_world.c -sWASM_ESM_INTEGRATION -sEXPORT_ES6 -O3 -o out.mjs

Result:

AssertionError [ERR_ASSERTION]: could not find the assignment to "wasmImports".
perhaps --pre-js or --post-js code moved it out of the global scope? ...
    at Object.<anonymous> (.../tools/acorn-optimizer.mjs:...)   // emitDCEGraph
emcc: error: '... acorn-optimizer.mjs <tmp>.js emitDCEGraph --no-print --export-es6' failed (returned 1)

The issue is that wasmImports is gated out under WASM_ESM_INTEGRATION in src/preamble.js:718.

The metadce needs to either be disabled or somehow be taught the ESM integration graph I believe here.

[sbc100]

Yes, I imagine metadce need to be taught about this alternative output format.

Hopefully its simpler output format which is more amenable to DCE in general. That was one of the original drivers for adding -sWASM_ESM_INTEGRATION in the first place: so the bundlers and tools can more clearly see the def/use graph.