No longer OOM on invalid string length.

src/hydrogen-instructions.cc

Index: src/hydrogen-instructions.cc
diff --git a/src/hydrogen-instructions.cc b/src/hydrogen-instructions.cc
index cfe5f1a60cd7f2a15ece7f75d3da2a989fb12168..dfa584a72ac228ee980ae516c17b6bc9e1c66652 100644
--- a/src/hydrogen-instructions.cc
+++ b/src/hydrogen-instructions.cc
@@ -3923,9 +3923,15 @@ HInstruction* HStringAdd::New(Zone* zone,
     HConstant* c_right = HConstant::cast(right);
     HConstant* c_left = HConstant::cast(left);
     if (c_left->HasStringValue() && c_right->HasStringValue()) {
-      Handle<String> concat = zone->isolate()->factory()->NewFlatConcatString(
-          c_left->StringValue(), c_right->StringValue());
-      return HConstant::New(zone, context, concat);
+      Handle<String> left_string = c_left->StringValue();
+      Handle<String> right_string = c_right->StringValue();
+      // Prevent possible exception by invalid string length.
+      if (left_string->length() + right_string->length() < String::kMaxLength) {
+        Handle<String> concat = zone->isolate()->factory()->NewFlatConcatString(
+            c_left->StringValue(), c_right->StringValue());
+        ASSERT(!concat.is_null());
+        return HConstant::New(zone, context, concat);
+      }
     }
   }
   return new(zone) HStringAdd(