FileSystem.walk is a wrapper for os.walk, which returns a generator object that recursively walks through subdirectories. For example, if we have a directory structure like this:

foo
├── bar
│   └── baz
├── a
├── b
└── c

Then `list(os.walk('foo'))` will return:

[
  ('foo', ['bar'], ['a', 'b', 'c']),
  ('foo/bar', [], ['baz'])
]


However, MockFileSystem.walk (https://code.google.com/p/chromium/codesearch#chromium/src/third_party/WebKit/Tools/Scripts/webkitpy/common/system/filesystem_mock.py&l=207) returns a list with one tuple, so it can't be correct in this case.