Remove support for operator negate and replace occurrences with unary

tests/language/condition_bailout_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 program testing closures.
 
 class A {
-  operator negate() => this;
+  operator -() => this;
 
   foo(x) {
     -a;
     if(x) return true;
     return false;
   }
 
   loop1(x) {
     -a;
     while(x) return true;
@@ skipping 18 matching lines @@
 
 var a;
 
 main() {
   a = new A();
   Expect.isTrue(a.foo(true));
   Expect.isTrue(a.loop1(true));
   Expect.isTrue(a.loop2(true));
   Expect.isTrue(a.loop3(true));
 }