Prevent monkey patching

This doesn't affect globals (describe, it, expect, etc). Those belong to
the user and Jasmine doesn't depend on them.

spec/helpers/monkeyPatchingSpecs.js

@@ -0,0 +1,67 @@
+globalThis.isNonMonkeyPatchableClass = function(ctor, makeInstance) {
+  describe('Monkey patching prevention', function() {
+    it(`prevents overwriting ${ctor.name}.prototype`, function() {
+      const existing = ctor.prototype;
+
+      try {
+        ctor.prototype = {};
+        expect(ctor.prototype).toBe(existing);
+      } finally {
+        // This will be a no-op if the test passed, but will prevent state
+        // leakage if it failed.
+        ctor.prototype = existing;
+      }
+    });
+
+    it("prevents overwriting an instance's prototype", function() {
+      const instance = makeInstance();
+      let thrown;
+
+      // The message varies from browser to browser, so we can't rely on it
+      try {
+        instance.__proto__ = {};
+      } catch (e) {
+        thrown = e;
+      }
+
+      expect(thrown).toBeInstanceOf(TypeError);
+    });
+
+    it('prevents overwriting prototype properties', function() {
+      let any = false;
+
+      for (const k of Object.getOwnPropertyNames(ctor.prototype)) {
+        any = true;
+        const existingValue = ctor.prototype[k];
+
+        try {
+          ctor.prototype[k] = {};
+          expect(ctor.prototype[k])
+            .withContext(k)
+            .toBe(existingValue);
+        } finally {
+          // This will be a no-op if the test passed, but will prevent state
+          // leakage if it failed.
+          ctor.prototype[k] = existingValue;
+        }
+      }
+
+      expect(any).toBe(true);
+    });
+
+    it('prevents overriding prototype properties', function() {
+      const instance = makeInstance();
+      let any = false;
+
+      for (const k of Object.getOwnPropertyNames(ctor.prototype)) {
+        any = true;
+        instance[k] = {};
+        expect(instance[k])
+          .withContext(k)
+          .toBe(ctor.prototype[k]);
+      }
+
+      expect(any).toBe(true);
+    });
+  });
+};