fix scope tracking for constants in loop heads - fixes #1229

src/babel/traversal/scope.js

@@ -449,7 +449,7 @@ export default class Scope {
     if (path.isLoop()) {
       for (let i = 0; i < t.FOR_INIT_KEYS.length; i++) {
         var node = path.get(t.FOR_INIT_KEYS[i]);
-        if (node.isBlockScoped()) this.registerBinding("let", node);
+        if (node.isBlockScoped()) this.registerBinding(node.node.kind, node);
       }
     }
 

test/core/fixtures/transformation/es6.constants/loop/actual.js

@@ -0,0 +1,3 @@
+for (const i = 0; i < 3; i = i + 1) {
+  console.log(i);
+}

test/core/fixtures/transformation/es6.constants/loop/options.json

@@ -0,0 +1,3 @@
+{
+  "throws": "Duplicate declaration \"i\""
+}