Sources for the CDK version that is included in this package can be obtained from https://github.com/cdk/cdk/commit/cb690f9f41378d742977af68aa8f1f77c20d55af. In general, if using Git, you can clone the repository corresponding to the version included here by doing 

git clone git@github.com:cdk/cdk.git

followed by 

git checkout cb690f9f41
