Migration: handle upcasts from function type to Function or Object.

This should address ~91 exceptions with this line in the stacktrace:

_AssignmentChecker._checkAssignment_recursion (package:nnbd_migration/src/edge_builder.dart:2031:7)

Change-Id: Id837ffb3eea207dd1f63010482a69397a8dff9ad
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/115768
Reviewed-by: Konstantin Shcheglov <scheglov@google.com>

pkg/nnbd_migration/lib/src/edge_builder.dart

@@ -2027,6 +2027,9 @@ mixin _AssignmentChecker {
       }
     } else if (destinationType.isDynamic || sourceType.isDynamic) {
       // ok; nothing further to do.
+    } else if (destinationType is InterfaceType && sourceType is FunctionType) {
+      // Either this is an upcast to Function or Object, or it is erroneous
+      // code.  In either case we don't need to create any additional edges.
     } else {
       throw '$destination <= $source'; // TODO(paulberry)
     }

pkg/nnbd_migration/test/edge_builder_test.dart

@@ -392,6 +392,15 @@ int f(dynamic d) => d;
         hard: true);
   }
 
+  test_assign_function_type_to_function_interface_type() async {
+    await analyze('''
+Function f(void Function() x) => x;
+''');
+    assertEdge(decoratedGenericFunctionTypeAnnotation('void Function()').node,
+        decoratedTypeAnnotation('Function f').node,
+        hard: true);
+  }
+
   test_assign_future_to_futureOr_complex() async {
     await analyze('''
 import 'dart:async';