JS calls with js_interop could be improved based on the expected type

[osa1]

import 'dart:js_interop';

@JS()
external String getString();

void main() {
  final string = getString();
}

Currently we generate this for getString:

(func $getString (;798;) (result (ref $Object))
  call $dart2wasm._311 (import)
  ref.null none
  call $dartifyRaw
  call $<obj> as String
  return)

dartifyRaw is a large function that considers all kinds of cases that will all result in an error:

sdk/sdk/lib/_internal/wasm/lib/js_helper.dart

Lines 461 to 493 in e87c93b

	Object? dartifyRaw(WasmExternRef? ref, [int? refType]) {
	refType ??= externRefType(ref);
	return switch (refType) {
	ExternRefType.null_ || ExternRefType.undefined => null,
	ExternRefType.boolean => toDartBool(ref),
	ExternRefType.number => toDartNumber(ref),
	ExternRefType.string => JSStringImpl.box(ref),
	ExternRefType.array => toDartList(ref),
	ExternRefType.int8Array => js_types.JSInt8ArrayImpl.fromJSArray(ref),
	ExternRefType.uint8Array => js_types.JSUint8ArrayImpl.fromJSArray(ref),
	ExternRefType.uint8ClampedArray => js_types
	.JSUint8ClampedArrayImpl.fromJSArray(ref),
	ExternRefType.int16Array => js_types.JSInt16ArrayImpl.fromJSArray(ref),
	ExternRefType.uint16Array => js_types.JSUint16ArrayImpl.fromJSArray(ref),
	ExternRefType.int32Array => js_types.JSInt32ArrayImpl.fromJSArray(ref),
	ExternRefType.uint32Array => js_types.JSUint32ArrayImpl.fromJSArray(ref),
	ExternRefType.float32Array => js_types.JSFloat32ArrayImpl.fromJSArray(ref),
	ExternRefType.float64Array => js_types.JSFloat64ArrayImpl.fromJSArray(ref),
	ExternRefType.arrayBuffer => js_types.JSArrayBufferImpl.fromRef(ref),
	ExternRefType.dataView => js_types.JSDataViewImpl.fromRef(ref),
	ExternRefType.unknown =>
	isJSWrappedDartFunction(ref)
	? unwrapJSWrappedDartFunction(ref)
	: isWasmGCStruct(ref)
	? jsObjectToDartObject(ref)
	: JSValue(ref),
	_ => () {
	// Assert that we've handled everything in the range.
	assert(refType! >= 0 && refType >= ExternRefType.unknown);
	throw 'Unhandled dartifyRaw type case: $refType';
	}(),
	};
	}

Instead of dartifyRaw, we could generate a dartifyString (or similar) that will only consider the string case, with improved type signature: (not tested)

JSStringImpl? dartifyString(WasmExternRef? ref) {
  refType ??= externRefType(ref);
  return switch (refType) {
    ExternRefType.string => JSStringImpl.box(ref),
    _ => null,
  }
}

This function is smaller and more efficient in the failing case. In the successful case it doesn't require a type test and ref.cast. It can be easily inlined as it's small, and inlining would allow follow-up optimizations when we do something like:

final s = getString();
final len = s.length;

Where inlining getString and length both would avoid boxing the externref in JStringImpl.

We should consider doing the same for all of the types that are allowed in js_interop.

[osa1]

#60357 is a duplicate of this one, but that one has more discussion now, so closing this one.