JS interop object literal constructors with extension types fail to compile when serializing codegen with sharding enabled

[srujzs]

import 'dart:js_interop';

extension type ObjectLiteral._(JSObject _) implements JSObject {
  external factory ObjectLiteral({int foo});
}

void main() {
  ObjectLiteral(foo: 0);
}

This should emit code that looks like {foo: 0}. We should visit the StaticInvocation in dart2js, and emit an HForeignCode in SSA. This exists today:

sdk/pkg/compiler/lib/src/ssa/builder.dart

Line 5804 in ae0510d

bool isObjectLiteralConstructor = node is ir.Procedure &&

and works when run directly. However, as part of its sequential phases, dart2js may serialize the codegen it produced. The code that does this sees the generated procedure for the above factory and mistakenly assumes it's a top-level interop procedure because it's external. Therefore, it results in a JS fragment like #.ObjectLiteral|constructor#(#) because it directly uses the name, which is invalid JS code, and therefore results in a crash.

The solution is to just skip the generated procedure.

One thing that still isn't clear though is why we don't need to do this for all the other generated procedures for interop extension type members. I believe it's because we replace all the invocations of such procedures in js_util_optimizer.dart whereas we defer the lowering for object literal constructors until later in dart2js. This may allow dart2js to tree-shake away those procedures early since there is no code referring to them anymore. Experimentally, that looks to be the case.

cc @rakudrama @stereotype441 @natebiggs