Add static loadJson method to Cesium3DTileset

Source/Scene/Cesium3DTileset.js

@@ -647,7 +647,7 @@ define([
         var that = this;
 
         // We don't know the distance of the tileset until tileset.json is loaded, so use the default distance for now
-        loadJson(tilesetUrl).then(function(tilesetJson) {
+        Cesium3DTileset.loadJson(tilesetUrl).then(function(tilesetJson) {
             that._root = that.loadTileset(tilesetUrl, tilesetJson);
             var gltfUpAxis = defined(tilesetJson.asset.gltfUpAxis) ? Axis.fromName(tilesetJson.asset.gltfUpAxis) : Axis.Y;
             that._asset = tilesetJson.asset;
@@ -1048,6 +1048,16 @@ define([
         }
     });
 
+    /**
+     * Provides a hook to override the method used to request the tileset json
+     * useful when fetching tilesets from remote servers
+     * @param {String} tilesetUrl The url of the json file to be fetched
+     * @returns {Promise.<Object>} A promise that resolves with the fetched json data
+     */
+    Cesium3DTileset.loadJson = function(tilesetUrl) {
+        return loadJson(tilesetUrl);
+    };
+
     /**
      * Marks the tileset's {@link Cesium3DTileset#style} as dirty, which forces all
      * features to re-evaluate the style in the next frame each is visible.

Specs/Scene/Cesium3DTilesetSpec.js

@@ -207,7 +207,48 @@ defineSuite([
         });
     });
 
-    it('rejects readyPromise with invalid tileset version', function() {
+    it('loads json with static loadJson method', function() {
+        var tilesetJson = {
+            asset : {
+                version : 2.0
+            }
+        };
+
+        var uri = 'data:text/plain;base64,' + btoa(JSON.stringify(tilesetJson));
+
+        Cesium3DTileset.loadJson(uri).then(function(result) {
+            expect(result).toEqual(tilesetJson);
+        }).otherwise(function(error) {
+            fail('should not fail');
+        });
+    });
+
+    it('static method loadJson is used in Cesium3DTileset constructor', function() {
+        var path = './Data/Cesium3DTiles/Tilesets/TilesetOfTilesets/tileset.json';
+
+        var originalLoadJson = Cesium3DTileset.loadJson;
+
+        // override loadJson and replace incorrect url with correct url
+        Cesium3DTileset.loadJson = function(tilesetUrl) {
+            return originalLoadJson(path);
+        }
+
+        // setup tileset with invalid url (overridden loadJson should replace invalid url with correct url)
+        var tileset = new Cesium3DTileset({
+            url : 'invalid.json'
+        });
+
+        // restore original version
+        Cesium3DTileset.loadJson = originalLoadJson;
+
+        return tileset.readyPromise.then(function() {
+            expect(tileset.ready).toEqual(true);
+        }).otherwise(function(error) {
+            fail('should not fail');
+        });
+    });
+
+     it('rejects readyPromise with invalid tileset version', function() {
         var tilesetJson = {
             asset : {
                 version : 2.0