[dart2js] new-rti: Make all closures have class type 'Closure'

Change-Id: I6713449b61a717907a5f8eac8fa3e2bea7e11ddd
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/111202
Commit-Queue: Mayank Patke <fishythefish@google.com>
Reviewed-by: Mayank Patke <fishythefish@google.com>

sdk/lib/_internal/js_runtime/lib/rti.dart

@@ -332,6 +332,18 @@ Rti instanceType(object) {
           'depends:none;effects:none;', JsBuiltin.dartObjectConstructor))) {
     var rti = JS('', r'#[#]', object, JS_GET_NAME(JsGetName.RTI_NAME));
     if (rti != null) return _castToRti(rti);
+
+    // Subclasses of Closure are synthetic classes, so make them appear to be
+    // the 'Closure' class.
+    // TODO(sra): Can this be done less expensively, e.g. by putting $ti on the
+    // prototype of Closure class?
+    var closureClassConstructor = JS_BUILTIN(
+        'depends:none;effects:none;', JsBuiltin.dartClosureConstructor);
+    if (closureClassConstructor != null &&
+        _Utils.instanceOf(object, closureClassConstructor)) {
+      return _instanceTypeFromConstructor(closureClassConstructor);
+    }
+
     return _instanceTypeFromConstructor(JS('', '#.constructor', object));
   }
 

tests/compiler/dart2js_extra/rti/simple_is_function_type3_test.dart

@@ -0,0 +1,31 @@
+// Copyright (c) 2019, 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.
+//
+// dart2jsOptions=--experiment-new-rti
+
+// Test that some closures are 'is Function'.
+
+import "package:expect/expect.dart";
+
+@pragma('dart2js:noInline')
+confuse(x) => x;
+
+main() {
+  // static tear-off.
+  Expect.isTrue(confuse(main) is Function);
+
+  // instance tear-off.
+  Expect.isTrue(confuse([].add) is Function);
+
+  // function expression.
+  Expect.isTrue(confuse(() => 1) is Function);
+
+  // local function.
+  int add1(int x) => x;
+
+  Expect.isTrue(confuse(add1) is Function);
+
+  Expect.isFalse(confuse(null) is Function);
+  Expect.isFalse(confuse(1) is Function);
+}