Matchers can have a negativeCompare

- Passing in a 'negativeCompare' will cause that function to be used when it is a 'not' assertion
- Otherwise, the reversal of the compare's result will be used instead

[finishes #59703824]

spec/core/CustomMatchersSpec.js

@@ -56,6 +56,28 @@ describe("Custom Matchers (Integration)", function() {
     env.execute();
   });
 
+  it("uses the negative compare function for a negative comparison, if provided", function(done) {
+    env.addMatchers({
+      toBeReal: function() {
+        return {
+          compare: function() { return { pass: true }; },
+          negativeCompare: function() { return { pass: true }; }
+        };
+      }
+    });
+
+    env.it("spec with custom negative comparison matcher", function() {
+      env.expect(true).not.toBeReal();
+    });
+
+    var specExpectations = function(result) {
+      expect(result.status).toEqual('passed');
+    }
+
+    env.addReporter({ specDone: specExpectations, jasmineDone: done });
+    env.execute();
+  });
+
   it("generates messages with the same rules as built in matchers absent a custom message", function(done) {
     env.addMatchers({
       toBeReal: function() {