Add type inference logic for map literals

pkg/front_end/testcases/inference/map_literals_top_level.dart.strong.expect

Index: pkg/front_end/testcases/inference/map_literals_top_level.dart.strong.expect
diff --git a/pkg/front_end/testcases/inference/map_literals_top_level.dart.strong.expect b/pkg/front_end/testcases/inference/map_literals_top_level.dart.strong.expect
new file mode 100644
index 0000000000000000000000000000000000000000..cc75346ac8291221408fc824f566b370928a8343
--- /dev/null
+++ b/pkg/front_end/testcases/inference/map_literals_top_level.dart.strong.expect
@@ -0,0 +1,23 @@
+library test;
+import self as self;
+import "dart:core" as core;
+
+static field core::Map<core::int, core::String> x1 = <core::int, core::String>{1: "x", 2: "y"};
+static field core::Map<core::num, core::Pattern> x2 = <core::num, core::Pattern>{1: "x", 2: "y", 3.0: core::RegExp::•(".")};
+static method test1() → dynamic {
+  self::x1.{core::Map::[]=}(3, "z");
+  self::x1.{core::Map::[]=}("hi", "w");
+  self::x1.{core::Map::[]=}(4.0, "u");
+  self::x1.{core::Map::[]=}(3, 42);
+  core::Map<core::num, core::String> y = self::x1;
+}
+static method test2() → dynamic {
+  self::x2.{core::Map::[]=}(3, "z");
+  self::x2.{core::Map::[]=}("hi", "w");
+  self::x2.{core::Map::[]=}(4.0, "u");
+  self::x2.{core::Map::[]=}(3, 42);
+  core::Pattern p = null;
+  self::x2.{core::Map::[]=}(2, p);
+  core::Map<core::int, core::String> y = self::x2;
+}
+static method main() → dynamic {}