Fix a duplicate AST ID recorded for for/in.

src/x64/full-codegen-x64.cc

Index: src/x64/full-codegen-x64.cc
diff --git a/src/x64/full-codegen-x64.cc b/src/x64/full-codegen-x64.cc
index 556ec85249bb9437a3fc244a2f617baeaa88dc15..4df0d5fbcedb44c26e433f1b6bfdc497255749f7 100644
--- a/src/x64/full-codegen-x64.cc
+++ b/src/x64/full-codegen-x64.cc
@@ -1686,11 +1686,21 @@ void FullCodeGenerator::EmitAssignment(Expression* expr, int bailout_ast_id) {
     }
     case KEYED_PROPERTY: {
       __ push(rax);  // Preserve value.
-      VisitForStackValue(prop->obj());
-      VisitForAccumulatorValue(prop->key());
-      __ movq(rcx, rax);
-      __ pop(rdx);
-      __ pop(rax);
+      if (prop->is_synthetic()) {
+        ASSERT(prop->obj()->AsVariableProxy() != NULL);
+        ASSERT(prop->key()->AsLiteral() != NULL);
+        { AccumulatorValueContext for_object(this);
+          EmitVariableLoad(prop->obj()->AsVariableProxy()->var());
+        }
+        __ movq(rdx, rax);
+        __ Move(rcx, prop->key()->AsLiteral()->handle());
+      } else {
+        VisitForStackValue(prop->obj());
+        VisitForAccumulatorValue(prop->key());
+        __ movq(rcx, rax);
+        __ pop(rdx);
+      }
+      __ pop(rax);  // Restore value.
       Handle<Code> ic(Builtins::builtin(Builtins::KeyedStoreIC_Initialize));
       EmitCallIC(ic, RelocInfo::CODE_TARGET);
       break;