make explode assignable expression helper aware of declared variables so we can simplify code a lot and drop the variable declaration as evaluating it multiple times has no consequence

lib/6to5/transformation/helpers/explode-assignable-expression.js

@@ -5,9 +5,25 @@ var t = require("../../types");
 var getObjRef = function (node, nodes, file, scope) {
   var ref;
   if (t.isIdentifier(node)) {
-    ref = node;
+    if (scope.has(node.name, true)) {
+      // this variable is declared in scope so we can be 100% sure
+      // that evaluating it multiple times wont trigger a getter
+      // or something else
+      return node;
+    } else {
+      // could possibly trigger a getter so we need to only evaluate
+      // it once
+      ref = node;
+    }
   } else if (t.isMemberExpression(node)) {
     ref = node.object;
+
+    if (t.isIdentifier(ref) && scope.has(ref.name)) {
+      // the object reference that we need to save is locally declared
+      // so as per the previous comment we can be 100% sure evaluating
+      // it multiple times will be safe
+      return ref;
+    }
   } else {
     throw new Error("We can't explode this node type " + node.type);
   }