Repository: ember-cli/ember-cli (js)

Issue:
Build/Serve changes are not detected for app.import files that have outputFile option
I have a file under `vendor` like so:  `vendor/shared.js` and this line in `ember-cli-build.js`:
```
app.import('vendor/shared.js', { outputFile: 'assets/shared-ui.js' });
```
`ember b` and `ember s` output the file correctly, but changes to the file while using `ember s` or `ember b -w` are not detected for this file.

Output from `ember version --verbose && npm --version`:
```
ember-cli: 2.8.0
http_parser: 2.7.0
node: 6.6.0
v8: 5.1.281.83
uv: 1.9.1
zlib: 1.2.8
ares: 1.10.1-DEV
icu: 57.1
modules: 48
openssl: 1.0.2h
os: darwin x64
3.10.3
```


Pull request description:
[BUGFIX] Watch vendor by default.
Closes #6431.

Patch:
diff --git a/lib/broccoli/ember-app.js b/lib/broccoli/ember-app.js
index cb6f6338b..4497a86fb 100644
--- a/lib/broccoli/ember-app.js
+++ b/lib/broccoli/ember-app.js
@@ -207,10 +207,11 @@ EmberApp.prototype._initOptions = function(options) {
   var templatesPath = this._resolveLocal('app/templates');
   var templatesTree = existsSync(templatesPath) ? new UnwatchedDir(templatesPath) : null;
 
-  // do not watch vendor/ or bower's default directory by default
+  // do not watch bower's default directory by default
   var bowerTree = this.project._watchmanInfo.enabled ? this.bowerDirectory : new UnwatchedDir(this.bowerDirectory);
+
   var vendorPath = this._resolveLocal('vendor');
-  var vendorTree = existsSync(vendorPath) ? new UnwatchedDir(vendorPath) : null;
+  var vendorTree = existsSync(vendorPath) ? new WatchedDir(vendorPath) : null;
 
   var publicPath = this._resolveLocal('public');
   var publicTree = existsSync(publicPath) ? new WatchedDir(publicPath) : null;


Test patch:
diff --git a/tests/fixtures/addon/simple/vendor/.gitkeep b/tests/fixtures/addon/simple/vendor/.gitkeep
new file mode 100644
index 000000000..e69de29bb
diff --git a/tests/unit/broccoli/ember-app-test.js b/tests/unit/broccoli/ember-app-test.js
index ff5007b2e..247b70f00 100644
--- a/tests/unit/broccoli/ember-app-test.js
+++ b/tests/unit/broccoli/ember-app-test.js
@@ -119,6 +119,14 @@ describe('broccoli/ember-app', function() {
       });
     });
 
+    it('should watch vendor if it exists', function() {
+      var app = new EmberApp({
+        project: project
+      });
+
+      expect(app.options.trees.vendor.__broccoliGetInfo__()).to.have.property('watched', true);
+    });
+
     describe('_notifyAddonIncluded', function() {
       beforeEach(function() {
         project.initializeAddons = function() { };