Move private APIs to private namespace

Fixes #2078

spec/core/matchers/NullDiffBuilderSpec.js

@@ -1,7 +1,7 @@
 describe('NullDiffBuilder', function() {
   it('responds to withPath() by calling the passed function', function() {
     const spy = jasmine.createSpy('callback');
-    jasmineUnderTest.NullDiffBuilder().withPath('does not matter', spy);
+    privateUnderTest.NullDiffBuilder().withPath('does not matter', spy);
     expect(spy).toHaveBeenCalled();
   });
 });