Implement new constant expressions

tests/language/compile_time_constant12_test.dart

Index: tests/language/compile_time_constant12_test.dart
diff --git a/tests/language/unary_plus_negative_test.dart b/tests/language/compile_time_constant12_test.dart
similarity index 76%
copy from tests/language/unary_plus_negative_test.dart
copy to tests/language/compile_time_constant12_test.dart
index 9f68d8ad7eb089d5a2a39d45f6fefcf5c3f8b156..cbe1a817c2ed2d7e91be447f410f298f2827dd37 100644
--- a/tests/language/unary_plus_negative_test.dart
+++ b/tests/language/compile_time_constant12_test.dart
@@ -2,9 +2,11 @@
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
-// There is no unary plus operator in Dart.
+const String s = "foo";
+const int i = s.length;
+
+use(x) => x;
 
 main() {
-  var a = 1;
-  var b = +a;
+  use(i);
 }