Impossible to use pseudo-parameters in CFN Mapping

[rsmogura]

Hi CDK team,

I recently tried to use following construct to get mapping parameter basing on account and region

let mappings = new cdk.Mapping(this, 'RegionsMap', {
            mapping: {
                "us-west-2": {
                    "000000000000": "..."
                }
            }
        });

mappings.findInMap(new cdk.AwsRegion(), new cdk.AwsAccountId());

However during execution I get exception (https://github.com/awslabs/aws-cdk/blob/master/packages/%40aws-cdk/cdk/lib/cloudformation/mapping.ts#L36) as findInMap tries to check if value is in map.

I wonder if mentioned check could be removed, as syntax with findInMap is shorter and less error prone than new cdk.FnFindInMap.

[eladb]

Makes sense of course. We could use cdk.unresolved to determine if the key is a concrete string or a token like {AWS::Region} and only check existence of the key if it's resolved