Add type inference logic for map literals

pkg/front_end/testcases/inference/map_literals_top_level.dart

Index: pkg/front_end/testcases/inference/map_literals_top_level.dart
diff --git a/pkg/front_end/testcases/inference/map_literals_top_level.dart b/pkg/front_end/testcases/inference/map_literals_top_level.dart
index 4352c2727a28ccca92812cd0f4a5259b243fc81e..7ce98e6c45bb393b442116455bb563a91fde6f4d 100644
--- a/pkg/front_end/testcases/inference/map_literals_top_level.dart
+++ b/pkg/front_end/testcases/inference/map_literals_top_level.dart
@@ -31,3 +31,5 @@ test2() {
   x2 /*@target=Map::[]=*/ [2] = p;
   Map<int, String> y = /*info:ASSIGNMENT_CAST*/ x2;
 }
+
+main() {}