Bug 778268 - Allow definitions with no uses to be removed. r=luke

js/src/frontend/Parser.cpp

@@ -782,7 +782,7 @@ Define(ParseNode *pn, JSAtom *atom, TreeContext *tc, bool let = false)
             pnup = &pnu->pn_link;
         }
 
-        if (pnu != dn->dn_uses) {
+        if (!pnu || pnu != dn->dn_uses) {
             *pnup = pn->dn_uses;
             pn->dn_uses = dn->dn_uses;
             dn->dn_uses = pnu;

js/src/jit-test/tests/basic/bug778268.js

@@ -0,0 +1,3 @@
+// Just don't assert
+j : 4;
+let [j] = 1, j;