Actually this is a blocker issue to me. Because from JSON you can't do:

node('windows') {
  retry(5) {
   steps...

Combined PR for JENKINS-41490 and JENKINS-41491 up at https://github.com/jenkinsci/pipeline-model-definition-plugin/pull/104

Code changed in jenkins
User: Andrew Bayer
Path:
pipeline-model-api/src/main/resources/ast-schema.json
pipeline-model-definition/src/main/groovy/org/jenkinsci/plugins/pipeline/modeldefinition/validator/ModelValidatorImpl.groovy
pipeline-model-definition/src/test/java/org/jenkinsci/plugins/pipeline/modeldefinition/AbstractModelDefTest.java
pipeline-model-definition/src/test/java/org/jenkinsci/plugins/pipeline/modeldefinition/BasicModelDefTest.java
pipeline-model-definition/src/test/resources/json/nestedTreeSteps.json
pipeline-model-definition/src/test/resources/nestedTreeSteps.groovy
http://jenkins-ci.org/commit/pipeline-model-definition-plugin/e4aae36cca402d8612e0c69110f9e7a9128f0ffc
Log:
[FIXED JENKINS-41490, JENKINS-41491] Tree steps and validation!

So this started with fixing JENKINS-41490, so that the editor can
actually do nested tree steps (d'oh), but in the process, I discovered
a strange decision from waaaaaay back in the day to force validation
in certain cases to treat the step parameter type as a String, even
when it wasn't one. That...was bad. So, fixing both those things.

Code changed in jenkins
User: Andrew Bayer
Path:
pipeline-model-api/src/main/resources/ast-schema.json
pipeline-model-definition/src/main/groovy/org/jenkinsci/plugins/pipeline/modeldefinition/validator/ModelValidatorImpl.groovy
pipeline-model-definition/src/test/java/org/jenkinsci/plugins/pipeline/modeldefinition/AbstractModelDefTest.java
pipeline-model-definition/src/test/java/org/jenkinsci/plugins/pipeline/modeldefinition/BasicModelDefTest.java
pipeline-model-definition/src/test/resources/json/nestedTreeSteps.json
pipeline-model-definition/src/test/resources/nestedTreeSteps.groovy
http://jenkins-ci.org/commit/pipeline-model-definition-plugin/ca9c202861194c80b78dd1f88c013643e524060a
Log:
Merge pull request #104 from abayer/jenkins-41490-jenkins-41491

[FIXED JENKINS-41490, JENKINS-41491] Tree steps and validation!

Compare: https://github.com/jenkinsci/pipeline-model-definition-plugin/compare/3a0ee9510ed4...ca9c20286119