[Core] Add 'now' service

Add a service to wrap Date.now(), such that this can be injected
(simplifying testing of scripts which need to respond to system
time.) Specifically supports staleness indication, WTD-660.

platform/core/bundle.json

@@ -148,6 +148,12 @@
                 "depends": [ "$q" ]
             }
         ],
+        "services": [
+            {
+                "key": "now",
+                "implementation": "services/Now.js"
+            }
+        ],
         "roots": [
             {
                 "id": "mine",

platform/core/src/services/Now.js

@@ -0,0 +1,27 @@
+/*global define*/
+
+define(
+    [],
+    function () {
+        "use strict";
+
+        /**
+         * Defines the `now` service, which is a simple wrapper upon
+         * `Date.now()` which can be injected to support testability.
+         *
+         * @returns {Function} a function which returns current system time
+         */
+        function Now() {
+            /**
+             * Get the current time.
+             * @returns {number} current time, in milliseconds since
+             *          1970-01-01 00:00:00Z
+             */
+            return function () {
+                return Date.now();
+            };
+        }
+
+        return Now;
+    }
+);

platform/core/test/services/NowSpec.js

@@ -0,0 +1,28 @@
+/*global define,Promise,describe,it,expect,beforeEach,waitsFor,jasmine*/
+
+define(
+    ["../../src/services/Now"],
+    function (Now) {
+        "use strict";
+
+        describe("The 'now' service", function () {
+            var now = new Now();
+
+            it("reports system time", function () {
+                var a = Date.now(),
+                    b = now(),
+                    c = Date.now();
+
+                // Clock could, in principle, tick between evaluating the
+                // expressions above. We can't predict or prevent this but
+                // want the test to be stable, so we only verify that now()
+                // returns a value that makes sense given a previous and
+                // subsequent measurement from Date.now()
+                expect(a <= b).toBeTruthy();
+                expect(b <= c).toBeTruthy();
+                expect(b).toBeDefined();
+            });
+
+        });
+    }
+);

platform/core/test/suite.json

@@ -20,6 +20,8 @@
     "objects/DomainObject",
     "objects/DomainObjectProvider",
 
+    "services/Now",
+
     "types/MergeModels",
     "types/TypeCapability",
     "types/TypeImpl",