Implement full closure inference logic.

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

+library test;
+import self as self;
+import "dart:core" as core;
+
+static method main() → dynamic {
+  () → (core::int) → core::double f = () → (core::int) → core::double {
+    return (core::int x) → core::double {
+      return 2.0.{core::double::*}(x);
+    };
+  };
+}