[dart2js] new-rti: Generate correct $signature functions for closures

Change-Id: I0f75195be4a75b46793b1cd9ec49ecc6e783611d
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/109723
Reviewed-by: Mayank Patke <fishythefish@google.com>
Commit-Queue: Stephen Adams <sra@google.com>

pkg/compiler/lib/src/ssa/builder_kernel.dart

@@ -514,7 +514,11 @@ class KernelSsaGraphBuilder extends ir.Visitor {
                 "Unexpected function signature: "
                 "$targetElement inside a non-closure: $target");
           }
-          _buildMethodSignature(originalClosureNode);
+          if (options.experimentNewRti) {
+            _buildMethodSignatureNewRti(originalClosureNode);
+          } else {
+            _buildMethodSignature(originalClosureNode);
+          }
           break;
         case MemberKind.generatorBody:
           _buildGeneratorBody(
@@ -1233,6 +1237,21 @@ class KernelSsaGraphBuilder extends ir.Visitor {
     _closeFunction();
   }
 
+  /// Constructs a special signature function for a closure.
+  void _buildMethodSignatureNewRti(ir.FunctionNode originalClosureNode) {
+    // The signature function has no corresponding ir.Node, so we just use the
+    // targetElement to set up the type environment.
+    _openFunction(targetElement, checks: TargetChecks.none);
+    FunctionType functionType =
+        _elementMap.getFunctionType(originalClosureNode);
+    HInstruction rti =
+        _typeBuilder.analyzeTypeArgumentNewRti(functionType, sourceElement);
+    close(new HReturn(_abstractValueDomain, rti,
+            _sourceInformationBuilder.buildReturn(originalClosureNode)))
+        .addSuccessor(graph.exit);
+    _closeFunction();
+  }
+
   /// Builds generative constructor body.
   void _buildConstructorBody(ir.Constructor constructor) {
     FunctionEntity constructorBody =

tests/compiler/dart2js_extra/rti/simple_is_function_type2_test.dart

@@ -0,0 +1,51 @@
+// 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
+
+import "package:expect/expect.dart";
+
+int fnInt2Int(int x) => x;
+int fnIntOptInt2Int(int x, [int y = 0]) => x + y;
+
+make1<A, B>(A a) {
+  return (B b) => a;
+}
+
+make2<A, B>(A a) {
+  A foo(B b) => a;
+  return foo;
+}
+
+class C<A> {
+  final A a;
+  C(this.a);
+  make<B>() => (B b) => a;
+  make2<B>() => (B b) => 'x';
+}
+
+main() {
+  Expect.isTrue(make1<int, int>(1) is int Function(int));
+  Expect.isTrue(make1<int, int>(1) is Object Function(int));
+  Expect.isTrue(make1<int, int>(1) is! String Function(int));
+  Expect.isTrue(make1<int, int>(1) is! int Function(String));
+
+  Expect.isTrue(make2<int, int>(1) is int Function(int));
+  Expect.isTrue(make2<int, int>(1) is Object Function(int));
+  Expect.isTrue(make2<int, int>(1) is! String Function(int));
+  Expect.isTrue(make2<int, int>(1) is! int Function(String));
+
+  Expect.isTrue(C<int>(1).make<String>() is int Function(String));
+  Expect.isTrue(C<int>(1).make<String>() is Object Function(String));
+  Expect.isTrue(C<int>(1).make<String>() is! String Function(int));
+
+  Expect.isTrue(C<int>(1).make2<int>() is String Function(int));
+  Expect.isTrue(C<int>(1).make2<int>() is Object Function(int));
+  Expect.isTrue(C<int>(1).make2<int>() is! int Function(String));
+  Expect.isTrue(C<int>(1).make2<int>() is! int Function(int));
+  Expect.isTrue(C<int>(1).make2<String>() is String Function(String));
+  Expect.isTrue(C<int>(1).make2<String>() is Object Function(String));
+  Expect.isTrue(C<int>(1).make2<String>() is! int Function(String));
+  Expect.isTrue(C<int>(1).make2<String>() is! String Function(int));
+}