[BUGFIX release] Enable linking ember-source without prod files.

Prior to these changes, in order to `npm link ember-source` you must
run a production build of Ember. This is because of the list of files
that we provide to `stew.find` (now `Funnel`) includes `ember.prod.js`
and `ember.min.js`.

This commit does two main things:

* Remove usage of `stew.find` (and therefore remove `broccoli-stew` as a dep).
  It is likely that `stew.find` will be deprecated (stefanpenner/broccoli-stew#122).
* Only provide the list of files that are present to the `Funnel` `files` listing.

Before this change the following would throw an error:

```
git clone [email protected]:emberjs/ember.js.git ember-canary
cd ember-canary
yarn install
./node_modules/.bin/ember build --environment=development
yarn link

cd ..
ember new test-app --yarn
cd test-app
yarn link ember-source
ember s
```

(cherry picked from commit 1a24de8)

index.js

@@ -1,5 +1,7 @@
 /* jshint node: true */
 'use strict';
+
+var fs = require('fs');
 var path = require('path');
 var resolve = require('resolve');
 
@@ -33,7 +35,8 @@ module.exports = {
   absolutePaths: absolutePaths,
 
   treeForVendor: function() {
-    var stew = require('broccoli-stew');
+    var Funnel = require('broccoli-funnel');
+    var MergeTrees = require('broccoli-merge-trees');
 
     var jqueryPath;
     try {
@@ -42,26 +45,29 @@ module.exports = {
       jqueryPath = path.dirname(require.resolve('jquery/package.json'));
     }
 
-    var jquery = stew.find(jqueryPath + '/dist', {
+    var jquery = new Funnel(jqueryPath + '/dist', {
       destDir: 'ember/jquery',
       files: [ 'jquery.js' ]
     });
 
-    var ember = stew.find(__dirname + '/dist', {
+    var emberFiles = [
+      'ember-runtime.js',
+      'ember-template-compiler.js',
+      'ember-testing.js',
+      'ember.debug.js',
+      'ember.min.js',
+      'ember.prod.js'
+    ].filter(function(file) {
+      var fullPath = path.join(__dirname, 'dist', file);
+
+      return fs.existsSync(fullPath);
+    });
+
+    var ember = new Funnel(__dirname + '/dist', {
       destDir: 'ember',
-      files: [
-        'ember-runtime.js',
-        'ember-template-compiler.js',
-        'ember-testing.js',
-        'ember.debug.js',
-        'ember.min.js',
-        'ember.prod.js'
-      ]
+      files: emberFiles
     });
 
-    return stew.find([
-      ember,
-      jquery
-    ]);
+    return new MergeTrees([ember, jquery]);
   }
 };

package.json

@@ -34,7 +34,8 @@
     "test:testem": "testem -f testem.dist.json"
   },
   "dependencies": {
-    "broccoli-stew": "^1.2.0",
+    "broccoli-funnel": "^1.0.6",
+    "broccoli-merge-trees": "^1.1.4",
     "ember-cli-get-component-path-option": "^1.0.0",
     "ember-cli-normalize-entity-name": "^1.0.0",
     "ember-cli-path-utils": "^1.0.0",
@@ -53,8 +54,6 @@
     "babel-plugin-filter-imports": "~0.2.0",
     "backburner.js": "^0.3.1",
     "bower": "~1.7.7",
-    "broccoli-funnel": "^1.0.6",
-    "broccoli-merge-trees": "^1.1.4",
     "broccoli-rollup": "^1.0.3",
     "broccoli-string-replace": "^0.1.1",
     "broccoli-uglify-sourcemap": "^1.4.2",