[pkg:js] Add tests for allowInterop idempotency

Closes #56897

Adds a few missing tests to make sure we handle already
converted functions, Dart functions that were wrapped
by a previous call, and JS functions correctly in
allowInterop and allowInteropCaptureThis.

Change-Id: Iaa6f34652717787fad8141ad941c47e944e70a5e
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/390400
Auto-Submit: Srujan Gaddam <srujzs@google.com>
Commit-Queue: Srujan Gaddam <srujzs@google.com>
Reviewed-by: Nicholas Shahan <nshahan@google.com>

tests/lib/js/allowInterop_idempotent_test.dart

@@ -0,0 +1,58 @@
+// Copyright (c) 2024, the Dart project authors.  Please see the AUTHORS file
+// for details. All rights reserved. Use of this source code is governed by a
+// BSD-style license that can be found in the LICENSE file.
+
+// Test that `allowInterop` and `allowInteropCaptureThis` should be idempotent
+// when given wrapped functions, JS functions, or Dart functions that were
+// wrapped by a previous call.
+
+import 'package:expect/expect.dart';
+import 'package:js/js.dart';
+
+const isDart2JS = const bool.fromEnvironment('dart.tool.dart2js');
+
+@JS()
+external void eval(String code);
+
+@JS()
+external Function get jsFunction;
+
+void main() {
+  eval('''
+    self.jsFunction = function() {};
+  ''');
+  final dartFunction = (_) {};
+  final wrappedFunction = allowInterop(dartFunction);
+  final wrappedFunctionCaptureThis = allowInteropCaptureThis(dartFunction);
+  // Should add a wrapper that's unique to the function.
+  Expect.notEquals(dartFunction, wrappedFunction);
+  Expect.notEquals(dartFunction, wrappedFunctionCaptureThis);
+  Expect.notEquals(wrappedFunction, wrappedFunctionCaptureThis);
+  // Should return the same value if the value is already a wrapped function.
+  Expect.equals(wrappedFunction, allowInterop(wrappedFunction));
+  Expect.equals(
+      wrappedFunctionCaptureThis, allowInterop(wrappedFunctionCaptureThis));
+  // Passing a non-Dart function to `allowInteropCaptureThis` throws in dart2js,
+  // so for now, capture the existing semantics. We may want to update DDC to
+  // match this.
+  if (isDart2JS) {
+    Expect.throws(() => allowInteropCaptureThis(wrappedFunction));
+    Expect.throws(() => allowInteropCaptureThis(wrappedFunctionCaptureThis));
+  } else {
+    Expect.equals(wrappedFunction, allowInteropCaptureThis(wrappedFunction));
+    Expect.equals(wrappedFunctionCaptureThis,
+        allowInteropCaptureThis(wrappedFunctionCaptureThis));
+  }
+  // Should directly return JS functions without doing anything.
+  Expect.equals(jsFunction, allowInterop(jsFunction));
+  if (isDart2JS) {
+    Expect.throws(() => allowInteropCaptureThis(jsFunction));
+  } else {
+    Expect.equals(jsFunction, allowInteropCaptureThis(jsFunction));
+  }
+  // If `allowInterop`/`allowInteropCaptureThis` is called again on the same
+  // Dart value, should return the previously wrapped function.
+  Expect.equals(wrappedFunction, allowInterop(dartFunction));
+  Expect.equals(
+      wrappedFunctionCaptureThis, allowInteropCaptureThis(dartFunction));
+}

tests/lib/lib.status

@@ -54,6 +54,7 @@ html/xhr_test/xhr: Skip # Times out.  Issue 21527
 [ $csp ]
 html/js_interop_constructor_name/*: SkipByDesign # Issue 42085.
 isolate/deferred_in_isolate2_test: Skip # Issue 16898. Deferred loading does not work from an isolate in CSP-mode
+js/allowInterop_idempotent_test: SkipByDesign # Issue 42085. CSP policy disallows injected JS code
 js/call_field_test: SkipByDesign # Issue 42085. CSP policy disallows injected JS code
 js/call_getter_test: SkipByDesign # Issue 42085. CSP policy disallows injected JS code
 js/call_method_test: SkipByDesign # Issue 42085. CSP policy disallows injected JS code