Allow binary operators on compile-time-constants.

frog/tests/leg_only/src/CompileTimeConstant4Test.dart

+// Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
+// 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.
+
+final x = "foo";
+final y = "foo";
+final g1 = x + "bar";
+final g2 = x
+  + null   /// 01: compile-time error
+;
+final g3 = x
+  + 499   /// 02: compile-time error
+;
+final g4 = x
+  + 3.3   /// 03: compile-time error
+;
+final g5 = x
+  + true   /// 04: compile-time error
+;
+final g6 = x
+  + false   /// 05: compile-time error
+;
+final g7 = "foo"
+  + x[0];   /// 06: compile-time error
+;
+final g8 = 1
+  + x.length  /// 07: compile-time error
+;
+final g9 = x == y;
+
+use(x) => x;
+
+main() {
+  Expect.equals("foobar", g1);
+  Expect.isTrue(g9);
+  use(g2);
+  use(g3);
+  use(g4);
+  use(g5);
+  use(g6);
+  use(g7);
+  use(g8);
+}