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-plugin-proposal-class-properties/src/index.js

@@ -25,6 +25,26 @@ export default function(api, options) {
     },
   };
 
+  const ClassFieldDefinitionEvaluationTDZVisitor = {
+    Expression(path) {
+      if (path === this.shouldSkip) {
+        path.skip();
+      }
+    },
+
+    ReferencedIdentifier(path) {
+      if (this.classRef === path.scope.getBinding(path.node.name)) {
+        const classNameTDZError = this.file.addHelper("classNameTDZError");
+        const throwNode = t.callExpression(classNameTDZError, [
+          t.stringLiteral(path.node.name),
+        ]);
+
+        path.replaceWith(t.sequenceExpression([throwNode, path.node]));
+        path.skip();
+      }
+    },
+  };
+
   const buildClassPropertySpec = (ref, { key, value, computed }, scope) => {
     return template.statement`
       Object.defineProperty(REF, KEY, {
@@ -95,6 +115,11 @@ export default function(api, options) {
           // Make sure computed property names are only evaluated once (upon class definition)
           // and in the right order in combination with static properties
           if (!computedPath.get("key").isConstantExpression()) {
+            computedPath.traverse(ClassFieldDefinitionEvaluationTDZVisitor, {
+              classRef: path.scope.getBinding(ref.name),
+              file: this.file,
+              shouldSkip: computedPath.get("value"),
+            });
             const ident = path.scope.generateUidIdentifierBasedOnNode(
               computedNode.key,
             );