fix(ngMock/$interval): add support for zero-delay intervals in tests

Previously, trying to test code thaat contained zero-delay intervals (e.g.
`$interval(fn, 0)` or `$interval(fn)`) would result in an infinite loop.
This commit avoids the infinite loop, by treating zero-delay intervals as one
second intervals (except for the initial trigger, where they can also be
executed with `$interval.flush(0)`).

Fixes #15952

Closes #15953

Author: luciomartinez

src/ngMock/angular-mocks.js

@@ -504,8 +504,8 @@ angular.mock.$IntervalProvider = function() {
       }
 
       repeatFns.push({
-        nextTime:(now + delay),
-        delay: delay,
+        nextTime: (now + (delay || 0)),
+        delay: delay || 1,
         fn: tick,
         id: nextRepeatId,
         deferred: deferred
@@ -555,10 +555,16 @@ angular.mock.$IntervalProvider = function() {
      * @return {number} The amount of time moved forward.
      */
     $interval.flush = function(millis) {
+      var before = now;
       now += millis;
       while (repeatFns.length && repeatFns[0].nextTime <= now) {
         var task = repeatFns[0];
         task.fn();
+        if (task.nextTime === before) {
+          // this can only happen the first time
+          // a zero-delay interval gets triggered
+          task.nextTime++;
+        }
         task.nextTime += task.delay;
         repeatFns.sort(function(a, b) { return a.nextTime - b.nextTime;});
       }

test/ngMock/angular-mocksSpec.js

@@ -351,6 +351,75 @@ describe('ngMock', function() {
     }));
 
 
+    it('should allow you to NOT specify the delay time', inject(function($interval) {
+      var counterA = 0;
+      var counterB = 0;
+
+      $interval(function() { counterA++; });
+      $interval(function() { counterB++; }, 0);
+
+      $interval.flush(1000);
+      expect(counterA).toBe(1000);
+      expect(counterB).toBe(1000);
+      $interval.flush(1000);
+      expect(counterA).toBe(2000);
+      expect(counterB).toBe(2000);
+    }));
+
+
+    it('should run tasks in correct relative order', inject(function($interval) {
+      var counterA = 0;
+      var counterB = 0;
+      $interval(function() { counterA++; }, 0);
+      $interval(function() { counterB++; }, 1000);
+
+      $interval.flush(1000);
+      expect(counterA).toBe(1000);
+      expect(counterB).toBe(1);
+      $interval.flush(999);
+      expect(counterA).toBe(1999);
+      expect(counterB).toBe(1);
+      $interval.flush(1);
+      expect(counterA).toBe(2000);
+      expect(counterB).toBe(2);
+    }));
+
+
+    it('should NOT trigger zero-delay interval when flush has ran before', inject(function($interval) {
+      var counterA = 0;
+      var counterB = 0;
+
+      $interval.flush(100);
+
+      $interval(function() { counterA++; });
+      $interval(function() { counterB++; }, 0);
+
+      expect(counterA).toBe(0);
+      expect(counterB).toBe(0);
+
+      $interval.flush(100);
+
+      expect(counterA).toBe(100);
+      expect(counterB).toBe(100);
+    }));
+
+
+    it('should trigger zero-delay interval only once on flush zero', inject(function($interval) {
+      var counterA = 0;
+      var counterB = 0;
+
+      $interval(function() { counterA++; });
+      $interval(function() { counterB++; }, 0);
+
+      $interval.flush(0);
+      expect(counterA).toBe(1);
+      expect(counterB).toBe(1);
+      $interval.flush(0);
+      expect(counterA).toBe(1);
+      expect(counterB).toBe(1);
+    }));
+
+
     it('should allow you to specify a number of iterations', inject(function($interval) {
       var counter = 0;
       $interval(function() {counter++;}, 1000, 2);