Ensure that a string is flattened when entering JsonParser::ParseJson. Fixes issue v8:1572.

src/json-parser.h

 // Copyright 2011 the V8 project authors. All rights reserved.
 // Redistribution and use in source and binary forms, with or without
 // modification, are permitted provided that the following conditions are
 // met:
 //
 //     * Redistributions of source code must retain the above copyright
 //       notice, this list of conditions and the following disclaimer.
 //     * Redistributions in binary form must reproduce the above
 //       copyright notice, this list of conditions and the following
 //       disclaimer in the documentation and/or other materials provided
@@ skipping 148 matching lines @@
   Handle<SeqAsciiString> seq_source_;
 
   Isolate* isolate_;
   uc32 c0_;
   int position_;
 };
 
 template <bool seq_ascii>
 Handle<Object> JsonParser<seq_ascii>::ParseJson(Handle<String> source) {
   isolate_ = source->map()->isolate();
-  source_ = Handle<String>(source->TryFlattenGetString());
+  FlattenString(source);
+  source_ = source;
   source_length_ = source_->length();
 
   // Optimized fast case where we only have ASCII characters.
   if (seq_ascii) {
     seq_source_ = Handle<SeqAsciiString>::cast(source_);
   }
 
   // Set initial position right before the string.
   position_ = -1;
   // Advance to the first character (posibly EOS)
@@ skipping 409 matching lines @@
   }
   ASSERT_EQ('"', c0_);
   // Advance past the last '"'.
   AdvanceSkipWhitespace();
   return result;
 }
 
 } }  // namespace v8::internal
 
 #endif  // V8_JSON_PARSER_H_