Add type inference logic for map literals

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

+library test;
+import self as self;
+import "dart:core" as core;
+
+typedef Function2 = <S extends core::Object, T extends core::Object>(S) → T;
+class A<T extends core::Object> extends core::Object {
+  field (self::A::T) → self::A::T x;
+  constructor •((self::A::T) → self::A::T x) → void
+    : self::A::x = x, super core::Object::•()
+    ;
+}
+static method main() → void {
+  {
+    core::String x = "hello";
+    core::int y = 3;
+    function f(core::List<core::Map<core::int, core::String>> l) → void {}
+    ;
+    f.call(<core::Map<core::int, core::String>>[<core::int, core::String>{y: x}]);
+  }
+  {
+    function f(core::int x) → core::int
+      return 0;
+    self::A<core::int> a = new self::A::•<core::int>(f);
+  }
+}