Call toDouble() in generated code

[xvrh]

This is the first step to fix: google/googleapis.dart#11

The generated code is now like:

if (_json.containsKey("property")) {
  property = _json["property"].toDouble();
}

// and

properties = commons.mapMap<core.num, core.double>(
    _json["properties"].cast<core.String, core.num>(),
     (core.num item) => item.toDouble());

[mkustermann]

/cc @jakobr-google

[mkustermann]

Thanks for the help @xavierhainaux , much appreciated!