Keep types in signatures of function types properly instantiated as the function

runtime/vm/dart_api_impl.cc

Index: runtime/vm/dart_api_impl.cc
diff --git a/runtime/vm/dart_api_impl.cc b/runtime/vm/dart_api_impl.cc
index 979f1b88f146478df4c0a95c901c6df12bcb12e6..212f270b6d4f5b5ae6e771a5f7b4b2035352718e 100644
--- a/runtime/vm/dart_api_impl.cc
+++ b/runtime/vm/dart_api_impl.cc
@@ -124,6 +124,11 @@ class CheckFunctionTypesVisitor : public ObjectVisitor {
     if (obj->IsFunction()) {
       funcHandle_ ^= obj;
       classHandle_ ^= funcHandle_.Owner();
+      // Signature functions get created, but not canonicalized, when function
+      // types get instantiated during run time type tests.
+      if (funcHandle_.IsSignatureFunction()) {
+        return;
+      }
       // Verify that the result type of a function is canonical or a
       // TypeParameter.
       typeHandle_ ^= funcHandle_.result_type();