build(broccoli): add support for DiffResult#addedPaths

Some plugins want to explicitly know of new paths, so we need to distinguish them from changed paths.

tools/broccoli/broccoli-flatten.spec.ts

@@ -52,4 +52,26 @@ describe('Flatten', () => {
 
     expect(fs.readdirSync('output')).toEqual(['file-1.1.txt', 'file-2.txt', 'file-3.txt']);
   });
+
+
+  it('should throw an exception if duplicates are found', () => {
+    let testDir = {
+      'input': {
+        'dir1': {
+          'file-1.txt': mockfs.file({content: 'file-1.txt content', mtime: new Date(1000)}),
+          'subdir-1': {
+            'file-1.txt': mockfs.file({content: 'file-1.1.txt content', mtime: new Date(1000)})
+          },
+          'empty-dir': {}
+        },
+      },
+      'output': {}
+    };
+    mockfs(testDir);
+
+    let differ = new TreeDiffer('testLabel', 'input');
+    let flattenedTree = flatten('input');
+    expect(() => flattenedTree.rebuild(differ.diffTree())).
+      toThrowError("Duplicate file 'file-1.txt' found in path 'dir1/subdir-1/file-1.txt'");
+  });
 });