Implement unary addition in the full (non-optimizing) code generator.

src/x64/full-codegen-x64.cc

Index: src/x64/full-codegen-x64.cc
===================================================================
--- src/x64/full-codegen-x64.cc	(revision 3706)
+++ src/x64/full-codegen-x64.cc	(working copy)
@@ -1464,6 +1464,20 @@
       break;
     }
 
+    case Token::ADD: {
+      Comment cmt(masm_, "[ UnaryOperation (ADD)");
+      VisitForValue(expr->expression(), kAccumulator);
+      Label no_conversion;
+      Condition is_smi;
+      is_smi = masm_->CheckSmi(result_register());
+      __ j(is_smi, &no_conversion);
+      __ push(result_register());
+      __ InvokeBuiltin(Builtins::TO_NUMBER, CALL_FUNCTION);
+      __ bind(&no_conversion);
+      Apply(context_, result_register());
+      break;
+    }
+
     default:
       UNREACHABLE();
   }