[dart2wasm] JS errors can't be caught using an interop type

[srujzs]

import 'dart:js_interop';

@JS()
external void eval(String code);

@JS()
external void throwFunction();

void main() {
  eval('''
    self.throwFunction = function() {
      throw new Error('hello world');
    }
  ''');
  try {
    throwFunction();
  } on JSObject catch (e) {
    print(e);
  }
}

DDC and dart2js catch the JS error as it is a subtype of interceptors.JavaScriptObject (and therefore a subtype of JSObject). dart2wasm does not, as its implementation of JS errors is not a subtype of JSObject:

sdk/sdk/lib/_internal/wasm/lib/errors_patch.dart

Line 40 in ad95bb6

class _JavaScriptError extends Error {

. A proposal is to make that implementation a subtype of JSValue, but it's not clear if a Wasm instruction exists to convert the error to an externref that can then be boxed. Making this a subtype of JSValue also has the other issue that if we replaced JSObject with JSString, dart2wasm would catch the error (because JSString is a JSValue) whereas DDC and dart2js will not.

For now, users should avoid catching JS errors using an interop type for consistency.

[srujzs]

Another alternative is simply linting/disallowing such a catch syntax as it relies on a runtime check of interop types.

[osa1]

but it's not clear if a Wasm instruction exists to convert the error to an externref that can then be boxed

This will be possible with WebAssembly/exception-handling#269. Chrome already supports it, but the latest Firefox (125.0.1) doesn't.