Fix for bug 1229: Unary operator plus is not allowed ... except for literals.

tests/language/src/LiteralUnaryPlusNegativeTest.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.
+
+// There is no unary plus operator in Dart.
+
+main() {
+  var a = + 1;

[hausner, 2012/01/31 18:09:46]

Would be nice to cover the negative test cases 
a = + "foo";
a = +"foo"; 
var x = +a;
x = + a;

[srdjan, 2012/01/31 19:26:37]

Added in a new test (removed this one).

+}