harmony-scoping: make assignment to 'const' a late error.

src/hydrogen.cc

Index: src/hydrogen.cc
diff --git a/src/hydrogen.cc b/src/hydrogen.cc
index 49d379aa0b018a977da1d520d4d91c9b3823acfb..1d33bfa6126c7ed6ed889440934b0835616f1fde 100644
--- a/src/hydrogen.cc
+++ b/src/hydrogen.cc
@@ -6647,6 +6647,9 @@ void HOptimizedGraphBuilder::HandleCompoundAssignment(Assignment* expr) {
         if (var->mode() == CONST_LEGACY)  {
           return Bailout(kUnsupportedConstCompoundAssignment);
         }
+        if (var->mode() == CONST) {
+          return Bailout(kNonInitializerAssignmentToConst);
+        }
         BindIfLive(var, Top());
         break;
 
@@ -6673,9 +6676,7 @@ void HOptimizedGraphBuilder::HandleCompoundAssignment(Assignment* expr) {
             mode = HStoreContextSlot::kCheckDeoptimize;
             break;
           case CONST:
-            // This case is checked statically so no need to
-            // perform checks here
-            UNREACHABLE();
+            return Bailout(kNonInitializerAssignmentToConst);
           case CONST_LEGACY:
             return ast_context()->ReturnValue(Pop());
           default:
@@ -10215,6 +10216,9 @@ void HOptimizedGraphBuilder::VisitCountOperation(CountOperation* expr) {
     if (var->mode() == CONST_LEGACY)  {
       return Bailout(kUnsupportedCountOperationWithConst);
     }
+    if (var->mode() == CONST) {
+      return Bailout(kNonInitializerAssignmentToConst);
+    }
     // Argument of the count operation is a variable, not a property.
     DCHECK(prop == NULL);
     CHECK_ALIVE(VisitForValue(target));