Remove support for operator negate and replace occurrences with unary

tests/language/operator_test.dart

 // Copyright (c) 2011, 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.
 
 class OperatorTest {
   static int i1, i2;
 
   OperatorTest() {}
 
   static testMain() {
@@ skipping 109 matching lines @@
   }
 
   operator >>(Operator other) {
     return value >> other.value;
   }
 
   operator ~/(Operator other) {
     return value ~/ other.value;
   }
 
-  operator negate() {
+  operator -() {
     return -value;
   }
 }
 
 main() {
   OperatorTest.testMain();
 }