Remove support for operator negate and replace occurrences with unary

tests/language/operator_test.dart

Index: tests/language/operator_test.dart
diff --git a/tests/language/operator_test.dart b/tests/language/operator_test.dart
index 5304f68c39fb0be6eb481b38ceb43e0407f3bffb..b692400e00559f893735cc2197597c4a1addd65a 100644
--- a/tests/language/operator_test.dart
+++ b/tests/language/operator_test.dart
@@ -127,7 +127,7 @@ class Operator {
     return value ~/ other.value;
   }
 
-  operator negate() {
+  operator -() {
     return -value;
   }
 }