When rewriting, mark newly constructed "this" nodes as non-indexable.

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=158038298

src/com/google/javascript/jscomp/Es6RewriteArrowFunction.java

@@ -159,6 +159,7 @@ private void addVarDecls(ThisContext thisContext) {
       Node name = IR.name(THIS_VAR);
       Node thisVar = IR.constNode(name, IR.thisNode());
       thisVar.useSourceInfoIfMissingFromForTree(scopeBody);
+      makeTreeNonIndexable(thisVar);
       if (thisContext.lastSuperStatement == null) {
         scopeBody.addChildToFront(thisVar);
       } else {
@@ -171,6 +172,13 @@ private void addVarDecls(ThisContext thisContext) {
     }
   }
 
+  private void makeTreeNonIndexable(Node n) {
+    n.makeNonIndexable();
+    for (Node child : n.children()) {
+      makeTreeNonIndexable(child);
+    }
+  }
+
   private static class UpdateThisAndArgumentsReferences implements NodeTraversal.Callback {
     private boolean changedThis = false;
     private boolean changedArguments = false;
@@ -184,6 +192,7 @@ public UpdateThisAndArgumentsReferences(AbstractCompiler compiler) {
     public void visit(NodeTraversal t, Node n, Node parent) {
       if (n.isThis()) {
         Node name = IR.name(THIS_VAR).srcref(n);
+        name.makeNonIndexable();
         if (compiler.getOptions().preservesDetailedSourceInfo()) {
           name.setOriginalName("this");
         }