Remove support for + prefix in number literals

tests/language/unary2_test.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.
 // Dart test for testing binary operations.
 
 import "package:expect/expect.dart";
 
 class UnaryTest {
   static foo() { return -4; }
-  static moo() { return +5; }
+  static moo() { return 5; }
   static testMain() {
     Expect.equals(1, (UnaryTest.foo() + UnaryTest.moo()));
-    var x = +.5;  // Legal number, '+' is not a unary operator.
   }
 }
 
 main() {
   UnaryTest.testMain();
 }