Add "platforms" key in IDL schema compiler.

tools/json_schema_compiler/model.py

Index: tools/json_schema_compiler/model.py
diff --git a/tools/json_schema_compiler/model.py b/tools/json_schema_compiler/model.py
index cc876dfad77730e9d72eaad52c73fff75b63fdf3..6e5ec55fade85cc21362628b6850c87411a5f3d0 100644
--- a/tools/json_schema_compiler/model.py
+++ b/tools/json_schema_compiler/model.py
@@ -486,7 +486,7 @@ class Platforms(object):
   WIN = _PlatformInfo("win")
 
 def _GetPlatforms(json):
-  if 'platforms' not in json:
+  if 'platforms' not in json or not json['platforms']:

[not at google - send to devlin, 2013/10/28 17:04:48]

if "platforms" can't be empty then this change doesn't do anything.

instead,

if 'platforms' not in json:
  return None
if not json['platforms']
  raise ValueError('"platforms" cannot be an empty list')

though just not changing this file at all is fine with me, too.

[Haojian Wu, 2013/10/29 02:09:32]

The statement "not json['platforms']"'s aim is to check whether the list is
None, instead empty list.

According to your comments, now I polish the code a little bit, and add empty
list check.

[not at google - send to devlin, 2013/10/29 14:36:52]

How is platforms None? There are three false states here: platforms not in the
JSON, platforms empty, and platforms set to None (or False or whatever). I don't
understand how the last of those could happen apart from just a wrong schema.

What breaks if you remove the json['platforms'] == None check?
Thanks.

[Haojian Wu, 2013/10/30 00:42:47]

The third false states: platforms set to None, there are two small cases, see
below:

1) If an IDL file with non-specific platfroms, like:

namespace Foo {...}

The json['platforms'] will be None.

2) If an IDL file with an empty platforms, like:

[platforms=()]
namespace Foo {...}

The json['platforms'] will be an empty list.

In the first case, _GetPlatforms should return None, while in the second case,
should raise an error based on your suggestions.

     return None
   platforms = []
   for platform_name in json['platforms']: