fix(ajax): AjaxTimeoutErrorImpl extends AjaxError (#5226)

* fix ajax: AjaxTimeoutError should extend AjaxError

Fixed AjaxTimeoutError not inheriting AjaxError's and Eerror's
prototype. While the TypeScript typings were correct, actual code
using instanceof operator would fail.

The following statements are now true:
* AjaxError instanceof Error === true
* AjaxTimeoutError instanceof Error === true
* AjaxTimeoutError instanceof AjaxError === true

* docs: CONTRIBUTING.ms was referencing Jasmine instead of Chai.

CONTRIBUTING.md

@@ -128,7 +128,7 @@ from the main (upstream) repository:
 ## Unit Tests
 
 Unit tests are located under the [spec directory](/spec). Unit tests over synchronous operators and operations
-can be written in a standard [jasmine](http://jasmine.github.io/) style. Unit tests written against any
+can be written in a standard [chai](https://www.chaijs.com/) style. Unit tests written against any
 asynchronous operator should be written in [Marble Test Style outlined in detail here](doc/writing-marble-tests.md).
 
 Each operator under test must be in its own file to cover the following cases:

spec/observables/dom/ajax-spec.ts

@@ -1082,6 +1082,27 @@ describe('ajax', () => {
       expect(request.headers).to.equal(headers);
     });
   });
+
+  describe('ajax error classes', () => {
+    describe('AjaxError', () => {
+      it('should extend Error class', () => {
+        const error = new AjaxError('Test error', new XMLHttpRequest(), {});
+        expect(error).to.be.an.instanceOf(Error);
+      });
+    });
+
+    describe('AjaxTimeoutError', () => {
+      it('should extend Error class', () => {
+        const error = new AjaxTimeoutError(new XMLHttpRequest(), {});
+        expect(error).to.be.an.instanceOf(Error);
+      });
+
+      it('should extend AjaxError class', () => {
+        const error = new AjaxTimeoutError(new XMLHttpRequest(), {});
+        expect(error).to.be.an.instanceOf(AjaxError);
+      });
+    });
+  });
 });
 
 class MockXMLHttpRequest {

src/internal/observable/dom/AjaxObservable.ts

@@ -536,11 +536,15 @@ export interface AjaxTimeoutErrorCtor {
   new(xhr: XMLHttpRequest, request: AjaxRequest): AjaxTimeoutError;
 }
 
-function AjaxTimeoutErrorImpl(this: any, xhr: XMLHttpRequest, request: AjaxRequest) {
-  AjaxError.call(this, 'ajax timeout', xhr, request);
-  this.name = 'AjaxTimeoutError';
-  return this;
-}
+const AjaxTimeoutErrorImpl = (() => {
+  function AjaxTimeoutErrorImpl(this: any, xhr: XMLHttpRequest, request: AjaxRequest) {
+    AjaxError.call(this, 'ajax timeout', xhr, request);
+    this.name = 'AjaxTimeoutError';
+    return this;
+  }
+  AjaxTimeoutErrorImpl.prototype = Object.create(AjaxError.prototype);
+  return AjaxTimeoutErrorImpl;
+})();
 
 /**
  * @see {@link ajax}