Add type inference logic for map literals

pkg/front_end/testcases/inference/infer_typed_map_literal.dart

Index: pkg/front_end/testcases/inference/infer_typed_map_literal.dart
diff --git a/pkg/front_end/testcases/inference/infer_typed_map_literal.dart b/pkg/front_end/testcases/inference/infer_typed_map_literal.dart
index 3ec80f1b985bfe9cb6601162652cc94e6efa7663..0b78431058115ade798cec0ddd63ad271011a0a8 100644
--- a/pkg/front_end/testcases/inference/infer_typed_map_literal.dart
+++ b/pkg/front_end/testcases/inference/infer_typed_map_literal.dart
@@ -12,3 +12,12 @@ var /*@topType=Map<List<int>, Map<String, double>>*/ c =
 var /*@topType=Map<int, dynamic>*/ d = <int, dynamic>{};
 var /*@topType=Map<dynamic, int>*/ e = <dynamic, int>{};
 var /*@topType=Map<dynamic, dynamic>*/ f = <dynamic, dynamic>{};
+
+main() {
+  a;
+  b;
+  c;
+  d;
+  e;
+  f;
+}