Supporting comments in build-cleaner.

build.json

 {
-  "#": "This file describes the list of targets and dependencies.",
-  "#": "It is used among other things to generate all of our project files.",
-  "#": "Please refer to the templates directory for more information.",
-
+  "#1": "This file describes the list of targets and dependencies.",
+  "#2": "It is used among other things to generate all of our project files.",
+  "#3": "Please refer to the templates directory for more information.",
   "settings": {
     "#": "The public version number of the library.",
     "version": {

tools/buildgen/build-cleaner.py

@@ -52,11 +52,15 @@ _ELEM_KEYS = [
 
 def rebuild_as_ordered_dict(indict, special_keys):
   outdict = collections.OrderedDict()
+  for key in sorted(indict.keys()):
+    if '#' in key:
+      outdict[key] = indict[key]
   for key in special_keys:
     if key in indict:
       outdict[key] = indict[key]
   for key in sorted(indict.keys()):
     if key in special_keys: continue
+    if '#' in key: continue
     outdict[key] = indict[key]
   return outdict