[js-api] Add tests for WebAssembly.JSTag (#319)

This adds tests for `WebAssembly.JSTag`. The tests are adapted from
https://github.com/v8/v8/blob/main/test/mjsunit/wasm/exnref-api.js.

Confirmed that they run successfully with web test infrastructure in
Chrome with `--js-flags=--experimental-wasm-exnref` command line
argument. The only thing failing there was the shadowrealm test, which I
think we should add a separate expectation files like the existing
`***.tentative.any.shadowrealm-expected.txt` in
https://github.com/chromium/chromium/tree/main/third_party/blink/web_tests/external/wpt/wasm/jsapi/exception.
But given that we don't host these files in this EH repo, I'll just
upload the js files.

test/js-api/exception/jsTag.tentative.any.js

@@ -0,0 +1,121 @@
+// META: global=window,dedicatedworker,jsshell,shadowrealm
+// META: script=/wasm/jsapi/assertions.js
+// META: script=/wasm/jsapi/wasm-module-builder.js
+
+test(() => {
+  assert_throws_js(TypeError, () => new WebAssembly.Exception(WebAssembly.JSTag, [{}]))
+}, "Creating a WebAssembly.Exception with JSTag explicitly is not allowed");
+
+promise_test(async () => {
+  const builder = new WasmModuleBuilder();
+  const jsTag = builder.addImportedTag("module", "JSTag", kSig_v_r);
+
+  const throwRefFn = builder.addImport("module", "throw_ref", kSig_r_r);
+  const sig_r_v = builder.addType(kSig_r_v);
+  const kSig_re_v = makeSig([], [kExternRefCode, kExnRefCode]);
+  const sig_re_v = builder.addType(kSig_re_v);
+
+  // Calls throw_ref, catches an exception with 'try_table - catch JSTag', and
+  // returns it
+  builder.addFunction("catch_js_tag_and_return", kSig_r_r)
+    .addBody([
+      kExprBlock, sig_r_v,
+        kExprTryTable, sig_r_v, 1,
+          kCatchNoRef, jsTag, 0,
+          kExprLocalGet, 0,
+          kExprCallFunction, throwRefFn,
+        kExprEnd,
+      kExprEnd,
+    ])
+    .exportFunc();
+
+  // Calls throw_ref, catches an exception with 'try_table - catch_ref JSTag',
+  // and returns it
+  builder.addFunction("catch_ref_js_tag_and_return", kSig_r_r)
+    .addBody([
+      kExprBlock, sig_re_v,
+        kExprTryTable, sig_r_v, 1,
+          kCatchRef, jsTag, 0,
+          kExprLocalGet, 0,
+          kExprCallFunction, throwRefFn,
+        kExprEnd,
+        kExprReturn,
+      kExprEnd,
+      kExprDrop,
+    ])
+    .exportFunc();
+
+  // Calls throw_ref, catches an exception with 'try_table - catch_ref JSTag',
+  // and rethrows it (with throw_ref)
+  builder.addFunction("catch_ref_js_tag_and_throw_ref", kSig_r_r)
+    .addBody([
+      kExprBlock, sig_re_v,
+        kExprTryTable, sig_r_v, 1,
+          kCatchRef, jsTag, 0,
+          kExprLocalGet, 0,
+          kExprCallFunction, throwRefFn,
+        kExprEnd,
+        kExprReturn,
+      kExprEnd,
+      kExprThrowRef,
+    ])
+    .exportFunc();
+
+  function throw_ref(x) {
+    throw x;
+  }
+  const buffer = builder.toBuffer();
+  const {instance} = await WebAssembly.instantiate(buffer, {
+    module: { throw_ref, JSTag: WebAssembly.JSTag }
+  });
+
+  const obj = {};
+  const wasmTag = new WebAssembly.Tag({parameters:['externref']});
+  const exn = new WebAssembly.Exception(wasmTag, [obj]);
+
+  // Test catch w/ return:
+  // This throws obj as a JS exception so it should be caught by the program and
+  // be returned as the original obj.
+  assert_equals(obj, instance.exports.catch_js_tag_and_return(obj));
+  // This is a WebAssembly.Exception, so the exception should just pass through
+  // the program without being caught.
+  assert_throws_exactly(exn, () => instance.exports.catch_js_tag_and_return(exn));
+
+  // Test catch_ref w/ return:
+  // This throws obj as a JS exception so it should be caught by the program and
+  // be returned as the original obj.
+  assert_equals(obj, instance.exports.catch_ref_js_tag_and_return(obj));
+  // This is a WebAssembly.Exception, so the exception should just pass through
+  // the program without being caught.
+  assert_throws_exactly(exn, () => instance.exports.catch_ref_js_tag_and_return(exn));
+
+  // Test catch_ref w/ throw_ref:
+  // This throws obj as a JS exception so it should be caught by the program and
+  // be rethrown.
+  assert_throws_exactly(obj, () => instance.exports.catch_ref_js_tag_and_throw_ref(obj));
+  // This is a WebAssembly.Exception, so the exception should just pass through
+  // the program without being caught.
+  assert_throws_exactly(exn, () => instance.exports.catch_ref_js_tag_and_throw_ref(exn));
+}, "JS tag catching tests");
+
+promise_test(async () => {
+  const builder = new WasmModuleBuilder();
+  const jsTag = builder.addImportedTag("module", "JSTag", kSig_v_r);
+
+  // Throw a JS object with WebAssembly.JSTag and check that we can catch it
+  // as-is from JavaScript.
+  builder.addFunction("throw_js_tag", kSig_v_r)
+    .addBody([
+      kExprLocalGet, 0,
+      kExprThrow, jsTag,
+    ])
+    .exportFunc();
+
+  const buffer = builder.toBuffer();
+  const {instance} = await WebAssembly.instantiate(buffer, {
+    module: { JSTag: WebAssembly.JSTag }
+  });
+
+  const obj = {};
+  assert_throws_exactly(obj, () => instance.exports.throw_js_tag(obj));
+}, "JS tag throwing test");