Infer return types of closures (basic support).

pkg/front_end/testcases/implicit_scope_test.dart.strong.expect

 library;
 import self as self;
 import "dart:core" as core;
 import "package:expect/expect.dart" as exp;
 
 class ImplicitScopeTest extends core::Object {
   constructor •() → void
     : super core::Object::•()
     ;
   static method alwaysTrue() → core::bool {
-    return 1.+(1).==(2);
+    return 1.{core::num::+}(1).{core::num::==}(2);
   }
   static method testMain() → dynamic {
     core::String a = "foo";
     dynamic b;
     if(self::ImplicitScopeTest::alwaysTrue()) {
       dynamic a = "bar";
     }
     else {
       dynamic b = a;
     }
@@ skipping 16 matching lines @@
       dynamic b = "baz";
     }
     while ("black".==("white"))
     exp::Expect::equals("foo", a);
     exp::Expect::equals(null, b);
   }
 }
 static method main() → dynamic {
   self::ImplicitScopeTest::testMain();
 }