Use official JSON encoding for stream

runtime/vm/json_stream.cc

Index: runtime/vm/json_stream.cc
diff --git a/runtime/vm/json_stream.cc b/runtime/vm/json_stream.cc
index f1a0d875d1e60ab9a0164ea12794ab2d3f1a9dd1..08f08edf6c2515cc9587de081fcf2a004109df6f 100644
--- a/runtime/vm/json_stream.cc
+++ b/runtime/vm/json_stream.cc
@@ -822,8 +822,14 @@ bool JSONStream::AddDartString(const String& s,
   }
   intptr_t limit = offset + count;
   for (intptr_t i = offset; i < limit; i++) {
-    intptr_t code_unit = s.CharAt(i);
-    buffer_.EscapeAndAddCodeUnit(code_unit);
+    uint32_t code_unit = s.CharAt(i);
+    ASSERT(!Utf16::IsTrailSurrogate(code_unit));
+    if (Utf16::IsLeadSurrogate(code_unit)) {
+      uint32_t trail = s.CharAt(++i);

[rmacnak, 2016/06/21 16:56:56]

A Dart string may have unmatched surrogates, so this might read after the end of
a Dart string or remove a character from the string.

[cbernaschina, 2016/06/21 21:24:43]

Done.

+      buffer_.EscapeAndAddCodeUnit(Utf16::Decode(code_unit, trail));
+    } else {
+      buffer_.EscapeAndAddCodeUnit(code_unit);
+    }
   }
   // Return value indicates whether the string is truncated.
   return (offset > 0) || (limit < length);