Error when compiling individual example

When compiling individual example with:
```
$ ./build.py build/examples/simple.combined.js
info ol Parsing dependencies
info ol Compiling 331 sources
ERR! compile ERROR - unknown @define variable goog.json.USE_NATIVE_JSON
ERR! compile 
ERR! compile 1 error(s), 0 warning(s), 98.0% typed
ERR! compile 
ERR! Process exited with non-zero status, see log for more detail: 1 
2014-09-17 15:58:18,346 build/examples/simple.combined.js: clean
2014-09-17 15:58:18,347 pake: build/examples/simple.combined.js: Command '['node', 'tasks/build.js', 'build/examples/simple.json', 'build/examples/simple.combined.js']' returned non-zero exit status 1
```

found by @elemoine