[dart2wasm] Simplify handling null receivers in gets, tear-offs, invocations

[osa1]

In dart2wasm we don't use the Null class for null values, instead we use Wasm's null value (ref.null).

This means we need special cases in the code generator when generating gets, tear-offs, and invocations (dynamic or instance) to check for null Wasm values and based on the member name generate code for null.toString tear-off or invocation, or null.hashCode instance get etc. Currently these look like this:

• Instance get:

  sdk/pkg/dart2wasm/lib/code_generator.dart

  Lines 1848 to 1872 in 50c5e8f

  	late w.Label doneLabel;
  	w.ValueType resultType =
  	_virtualCall(node, target, _VirtualCallKind.Get, (signature) {
  	doneLabel = b.block(const [], signature.outputs);
  	w.Label nullLabel = b.block();
  	wrap(node.receiver, translator.topInfo.nullableType);
  	b.br_on_null(nullLabel);
  	}, (_) {});
  	b.br(doneLabel);
  	b.end(); // nullLabel
  	switch (target.name.text) {
  	case "hashCode":
  	b.i64_const(2011);
  	break;
  	case "runtimeType":
  	case "_runtimeType":
  	wrap(ConstantExpression(TypeLiteralConstant(NullType())), resultType);
  	break;
  	default:
  	unimplemented(
  	node, "Nullable get of ${target.name.text}", [resultType]);
  	break;
  	}
  	b.end(); // doneLabel
  	return resultType;

• Instance tear-off:

  sdk/pkg/dart2wasm/lib/code_generator.dart

  Lines 1928 to 1959 in 50c5e8f

  	late w.Label doneLabel;
  	w.ValueType resultType =
  	_virtualCall(node, target, _VirtualCallKind.Get, (signature) {
  	doneLabel = b.block(const [], signature.outputs);
  	w.Label nullLabel = b.block();
  	wrap(node.receiver, translator.topInfo.nullableType);
  	b.br_on_null(nullLabel);
  	translator.convertType(
  	function, translator.topInfo.nullableType, signature.inputs[0]);
  	}, (_) {});
  	b.br(doneLabel);
  	b.end(); // nullLabel
  	switch (target.name.text) {
  	case "toString":
  	wrap(
  	ConstantExpression(
  	StaticTearOffConstant(translator.nullToString)),
  	resultType);
  	break;
  	case "noSuchMethod":
  	wrap(
  	ConstantExpression(
  	StaticTearOffConstant(translator.nullNoSuchMethod)),
  	resultType);
  	break;
  	default:
  	unimplemented(
  	node, "Nullable tear-off of ${target.name.text}", [resultType]);
  	break;
  	}
  	b.end(); // doneLabel
  	return resultType;

• Instance invocation:

  sdk/pkg/dart2wasm/lib/code_generator.dart

  Lines 1455 to 1476 in 50c5e8f

  	switch (target.name.text) {
  	case "toString":
  	late w.Label done;
  	w.ValueType resultType =
  	_virtualCall(node, target, _VirtualCallKind.Call, (signature) {
  	done = b.block(const [], signature.outputs);
  	w.Label nullString = b.block();
  	wrap(node.receiver, translator.topInfo.nullableType);
  	b.br_on_null(nullString);
  	}, (_) {
  	_visitArguments(node.arguments, node.interfaceTargetReference, 1);
  	});
  	b.br(done);
  	b.end();
  	wrap(StringLiteral("null"), resultType);
  	b.end();
  	return resultType;
  	default:
  	unimplemented(node, "Nullable invocation of ${target.name.text}",
  	[if (expectedType != voidMarker) expectedType]);
  	return expectedType;
  	}

• There should also be a few lines in dynamic invocation code generator

Since we don't use an actual Dart object for null we can't completely avoid these special cases, but we can simplify them a little bit by having a class representing the null object (say, WasmNull), and when a receiver is null in a get, tear-off etc. we replace the receiver with the singleton instance of WasmNull. This class can be defined in Dart with the right toString, hashCode etc. implementations.

I tried this in a branch but it caused various crashes and it wasn't immediately obvious how can my changes break that much. This won't fix any tests but it may be a worthwhile simplification. We should probably revisit this at some point.

Related CLs:

• https://dart-review.googlesource.com/c/sdk/+/268462 tries to use the core's Null class.
• https://dart-review.googlesource.com/c/sdk/+/266627 uses a new class WasmNull instead of core's Null.

[osa1]

Without a null object, the special cases in instance invocations above need to also handle noSuchMethod.

[osa1]

The test tests/language/closure/tearoff_dynamic_test fails because of missing null.noSuchMethod handling.

[mkustermann]

The test tests/language/closure/tearoff_dynamic_test fails because of missing null.noSuchMethod handling.

This test seems to be passing now.