better categorisation of transformers

lib/6to5/transformation/transformers/es6/block-scoping-tdz.js

@@ -0,0 +1,48 @@
+var traverse = require("../../../traverse");
+var t        = require("../../../types");
+
+var visitor = {
+  enter: function (node, parent, scope, context, state) {
+    if (!t.isReferencedIdentifier(node, parent)) return;
+
+    var declared = state.letRefs[node.name];
+    if (!declared) return;
+
+    // declared node is different in this scope
+    if (scope.get(node.name, true) !== declared) return;
+
+    var declaredLoc  = declared.loc;
+    var referenceLoc = node.loc;
+
+    if (!declaredLoc || !referenceLoc) return;
+
+    // does this reference appear on a line before the declaration?
+    var before = referenceLoc.start.line < declaredLoc.start.line;
+
+    if (referenceLoc.start.line === declaredLoc.start.line) {
+      // this reference appears on the same line
+      // check it appears before the declaration
+      before = referenceLoc.start.col < declaredLoc.start.col;
+    }
+
+    if (before) {
+      throw state.file.errorWithNode(node, "Temporal dead zone - accessing a variable before it's initialized");
+    }
+  }
+};
+
+exports.optional = true;
+
+exports.Loop =
+exports.Program =
+exports.BlockStatement = function (node, parent, scope, context, file) {
+  var letRefs = node._letReferences;
+  if (!letRefs) return;
+
+  var state = {
+    letRefs: letRefs,
+    file:    file
+  };
+
+  traverse(node, visitor, scope, state);
+};