[dart2wasm] Debugging information for release builds (e.g. dwarf, source maps)

[mkustermann]

For release builds we are stripping the static name section from the wasm by-default. For local debugging one can compile with name section included (e.g. flutter build web --wasm --no-strip-wasm) but apps are usually deployed without symbols to reduce the wasm file size.

This means stack traces at runtime are non-symbolic. There's several ways one could get symbolic stack traces:

• Make stripping the name section a separate pass, remember the mapping, provide offline decoding tool
• Emit source maps in the compiler, which binaryen preserves across optimizations
• Emit dwarf in the compiler, which binaryen may preserve to some extent

I stumbled upon this code in the binaryen sources in pass.cpp:

void PassRunner::addDefaultFunctionOptimizationPasses() {
  // All the additions here are optional if DWARF must be preserved. That is,
  // when DWARF is relevant we run fewer optimizations.
  // FIXME: support DWARF in all of them.
  ...

Because of that, I think we should avoid DWARF for the time being and instead emit source maps.

[mkustermann]

/cc @osa1

[osa1]

I started working on generating source maps.

[mkustermann]

The source map support has landed now in 10742d9. Closing this issue.

I'll file another issue with some follow-up tasks.