Docserver: Add more skip_not_found support and cache "not found"s

chrome/common/extensions/docs/server2/file_system.py

Index: chrome/common/extensions/docs/server2/file_system.py
diff --git a/chrome/common/extensions/docs/server2/file_system.py b/chrome/common/extensions/docs/server2/file_system.py
index e459e1207c155e63101944bd429ad54ef6b3299c..e8209934c35b700358764b00fcf467747fd1489d 100644
--- a/chrome/common/extensions/docs/server2/file_system.py
+++ b/chrome/common/extensions/docs/server2/file_system.py
@@ -84,13 +84,13 @@ class FileSystem(object):
     '''
     raise NotImplementedError(self.__class__)
 
-  def ReadSingle(self, path):
+  def ReadSingle(self, path, skip_not_found=False):
     '''Reads a single file from the FileSystem. Returns a Future with the same
     rules as Read(). If |path| is not found raise a FileNotFoundError on Get().
     '''
     AssertIsValid(path)
-    read_single = self.Read([path])
-    return Future(callback=lambda: read_single.Get()[path])
+    read_single = self.Read([path], skip_not_found=skip_not_found)
+    return Future(callback=lambda: read_single.Get().get(path, None))
 
   def Exists(self, path):
     '''Returns a Future to the existence of |path|; True if |path| exists,