[strong] Disallow implicit conversions for add

src/hydrogen.cc

Index: src/hydrogen.cc
diff --git a/src/hydrogen.cc b/src/hydrogen.cc
index 937f5b086b41029a1e4c70130ae3953cca3d1fe8..f6a6d8d147d356129bd17f80d6a200695b9899e4 100644
--- a/src/hydrogen.cc
+++ b/src/hydrogen.cc
@@ -10611,23 +10611,27 @@ HValue* HGraphBuilder::BuildBinaryOperation(
     }
 
     // Convert left argument as necessary.
-    if (left_type->Is(Type::Number())) {
+    if (left_type->Is(Type::Number()) && !is_strong(language_mode)) {
       DCHECK(right_type->Is(Type::String()));
       left = BuildNumberToString(left, left_type);
     } else if (!left_type->Is(Type::String())) {
       DCHECK(right_type->Is(Type::String()));
-      HValue* function = AddLoadJSBuiltin(Builtins::STRING_ADD_RIGHT);
+      HValue* function = AddLoadJSBuiltin(is_strong(language_mode) ?
+                                            Builtins::STRING_ADD_RIGHT_STRONG :
+                                            Builtins::STRING_ADD_RIGHT);
       Add<HPushArguments>(left, right);
       return AddUncasted<HInvokeFunction>(function, 2);
     }
 
     // Convert right argument as necessary.
-    if (right_type->Is(Type::Number())) {
+    if (right_type->Is(Type::Number()) && !is_strong(language_mode)) {
       DCHECK(left_type->Is(Type::String()));
       right = BuildNumberToString(right, right_type);
     } else if (!right_type->Is(Type::String())) {
       DCHECK(left_type->Is(Type::String()));
-      HValue* function = AddLoadJSBuiltin(Builtins::STRING_ADD_LEFT);
+      HValue* function = AddLoadJSBuiltin(is_strong(language_mode) ?
+                                            Builtins::STRING_ADD_LEFT_STRONG :
+                                            Builtins::STRING_ADD_LEFT);
       Add<HPushArguments>(left, right);
       return AddUncasted<HInvokeFunction>(function, 2);
     }