Add another ShadowRealm test for onerror

This test is similar to window-onerror-runtime-error-throw.html and
window-onerror-runtime-error.html, but ensuring that the ShadowRealm
global's onerror is triggered and not the Window's onerror.

By my reading of the spec, we cannot have a test equivalent to
window-onerror-parse-error.html in ShadowRealm because
ShadowRealm.prototype.evaluate() will exit early if the given code fails
to parse.

html/webappapis/scripting/processing-model-2/globalthis-onerror-shadowrealm.window.js

@@ -0,0 +1,71 @@
+// META: title=globalThis.onerror: runtime script errors in ShadowRealm
+
+// https://html.spec.whatwg.org/multipage/#runtime-script-errors says what to do
+// for uncaught runtime script errors, and just below describes what to do when
+// onerror is a Function.
+
+async_test(t => {
+  onerror = t.unreached_func("Window onerror should not be triggered");
+
+  const realm = new ShadowRealm();
+
+  realm.evaluate("var errorCount = 0;");
+  realm.evaluate(`(doAsserts) => {
+    globalThis.onerror = function(msg, url, lineno, colno, thrownValue) {
+      ++errorCount;
+      doAsserts(url, lineno, colno, typeof thrownValue, String(thrownValue));
+    };
+  }`)(t.step_func((url, lineno, typeofThrownValue, stringifiedThrownValue) => {
+    assert_equals(url, "eval code", "correct url passed to onerror");
+    assert_equals(lineno, 8, "correct line number passed to onerror");
+    assert_equals(typeofThrownValue, "string", "thrown string passed directly to onerror");
+    assert_equals(stringifiedThrownValue, "bar", "correct thrown value passed to onerror");
+  }));
+
+  assert_throws_js(TypeError, () => realm.evaluate(`
+    try {
+      // This error is caught, so it should NOT trigger onerror.
+      throw "foo";
+    } catch (ex) {
+    }
+    // This error is NOT caught, so it should trigger onerror.
+    throw "bar";
+  `), "thrown error is wrapped in a TypeError object from the surrounding realm");
+
+  t.step_timeout(() => {
+    assert_equals(realm.evaluate("errorCount"), 1, "onerror should be called once");
+  }, 1000);
+}, "onerror triggered by uncaught thrown exception in realm.evaluate");
+
+async_test(t => {
+  onerror = t.unreached_func("Window onerror should not be triggered");
+
+  const realm = new ShadowRealm();
+
+  realm.evaluate("var errorCount = 0;");
+  realm.evaluate(`(doAsserts) => {
+    globalThis.onerror = function(msg, url, lineno, colno, thrownValue) {
+      ++errorCount;
+      doAsserts(url, lineno, typeof thrownValue, thrownValue instanceof TypeError);
+    };
+  }`)(t.step_func((url, lineno, typeofThrownValue, isTypeError) => {
+    assert_equals(url, "eval code", "correct url passed to onerror");
+    assert_equals(lineno, 8, "correct line number passed to onerror");
+    assert_equals(typeofThrownValue, "object", "thrown error instance passed to onerror");
+    assert_true(isShadowRealmTypeError, "correct thrown value passed to onerror");
+  }));
+
+  assert_throws_js(TypeError, () => realm.evaluate(`
+    try {
+      // This error is caught, so it should NOT trigger onerror.
+      window.nonexistentproperty.oops();
+    } catch (ex) {
+    }
+    // This error is NOT caught, so it should trigger onerror.
+    window.nonexistentproperty.oops();
+  `), "thrown error is wrapped in a TypeError object from the surrounding realm");
+
+  t.step_timeout(() => {
+    assert_equals(realm.evaluate("errorCount"), 1, "onerror should be called once");
+  }, 1000);
+}, "onerror triggered by uncaught runtime error in realm.evaluate");