Remove support for operator negate and replace occurrences with unary

lib/core/num.dart

Index: lib/core/num.dart
diff --git a/lib/core/num.dart b/lib/core/num.dart
index 1ef36131879bcc1f9c72aadc4cdb63bb919932be..2b3e263a8be8d79aef642efba47b5cc5cfd215ba 100644
--- a/lib/core/num.dart
+++ b/lib/core/num.dart
@@ -14,7 +14,7 @@ abstract class num implements Comparable, Hashable {
   // Truncating division.
   abstract num operator ~/(num other);
   // The unary '-' operator.
-  abstract num operator negate();
+  abstract num operator -();
   abstract num remainder(num other);
 
   // Relational operations.