Use official JSON encoding for stream

runtime/vm/json_stream.cc

 // Copyright (c) 2013, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 #include "platform/assert.h"
 
 #include "vm/dart_entry.h"
 #include "vm/debugger.h"
 #include "vm/json_stream.h"
 #include "vm/message.h"
@@ skipping 804 matching lines @@
   intptr_t length = s.Length();
   ASSERT(offset >= 0);
   if (offset > length) {
     offset = length;
   }
   if (!Utils::RangeCheck(offset, count, length)) {
     count = length - offset;
   }
   intptr_t limit = offset + count;
   for (intptr_t i = offset; i < limit; i++) {
-    intptr_t code_unit = s.CharAt(i);
-    buffer_.EscapeAndAddCodeUnit(code_unit);
+    uint16_t code_unit = s.CharAt(i);
+    if (Utf16::IsTrailSurrogate(code_unit)) {
+      buffer_.EscapeAndAddUTF16CodeUnit(code_unit);
+    } else if (Utf16::IsLeadSurrogate(code_unit)) {
+      if (i + 1 == limit) {
+        buffer_.EscapeAndAddUTF16CodeUnit(code_unit);
+      } else {
+        uint16_t next_code_unit = s.CharAt(i+1);
+        if (Utf16::IsTrailSurrogate(next_code_unit)) {
+          uint32_t decoded = Utf16::Decode(code_unit, next_code_unit);
+          buffer_.EscapeAndAddCodeUnit(decoded);
+          i++;
+        } else {
+          buffer_.EscapeAndAddUTF16CodeUnit(code_unit);
+        }
+      }
+    } else {
+      buffer_.EscapeAndAddCodeUnit(code_unit);
+    }
   }
   // Return value indicates whether the string is truncated.
   return (offset > 0) || (limit < length);
 }
 
 
 JSONObject::JSONObject(const JSONArray* arr) : stream_(arr->stream_) {
   stream_->OpenObject();
 }
 
@@ skipping 108 matching lines @@
   ASSERT(len == len2);
   stream_->buffer_.AddChar('"');
   stream_->AddEscapedUTF8String(p);
   stream_->buffer_.AddChar('"');
   free(p);
 }
 
 #endif  // !PRODUCT
 
 }  // namespace dart