Implement TDZ for ClassFieldDefinitionEvaluation (#6855)

* First implementation sketch and testcase

* Use helper instead of inline IIFE's

* minNodeVersion 6.0.0

* Hoisted visitor for subtraversal and fixed edgest case

* Resolve merge conflicts in helpers

* Remove duplicated helper from messing up Git

packages/babel-helpers/src/helpers.js

@@ -633,6 +633,12 @@ helpers.readOnlyError = defineHelper(`
   }
 `);
 
+helpers.classNameTDZError = defineHelper(`
+  export default function _classNameTDZError(name) {
+    throw new Error("Class \\"" + name + "\\" cannot be referenced in computed property keys.");
+  }
+`);
+
 helpers.temporalUndefined = defineHelper(`
   export default {};
 `);