add no loopParent warning to block scoping transformer

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

@@ -454,6 +454,11 @@ LetScoping.prototype.buildHas = function (ret, call) {
   }
 
   if (has.hasBreak || has.hasContinue) {
+    if (!loopParent) {
+      throw new Error("Has no loop parent but we're trying to reassign breaks " +
+                      "and continues, something is going wrong here.");
+    }
+
     // ensure that the parent has a label as we're building a switch and we
     // need to be able to access it
     var label = loopParent.label = loopParent.label || this.scope.generateUidIdentifier("loop");