Add support to trace warnings in TraceBuffer.

runtime/vm/json_stream.h

Index: runtime/vm/json_stream.h
===================================================================
--- runtime/vm/json_stream.h	(revision 37297)
+++ runtime/vm/json_stream.h	(working copy)
@@ -83,6 +83,7 @@
   void PrintValue64(int64_t i);
   void PrintValue(double d);
   void PrintValue(const char* s);
+  void PrintValueNoEscape(const char* s);
   void PrintfValue(const char* format, ...) PRINTF_ATTRIBUTE(2, 3);
   void PrintValue(const Object& o, bool ref = true);
   void PrintValue(SourceBreakpoint* bpt);
@@ -94,6 +95,7 @@
   void PrintProperty64(const char* name, int64_t i);
   void PrintProperty(const char* name, double d);
   void PrintProperty(const char* name, const char* s);
+  void PrintPropertyNoEscape(const char* name, const char* s);
   void PrintfProperty(const char* name, const char* format, ...)
   PRINTF_ATTRIBUTE(3, 4);
   void PrintProperty(const char* name, const Object& o, bool ref = true);
@@ -153,6 +155,9 @@
   void AddProperty(const char* name, const char* s) const {
     stream_->PrintProperty(name, s);
   }
+  void AddPropertyNoEscape(const char* name, const char* s) const {
+    stream_->PrintPropertyNoEscape(name, s);
+  }
   void AddProperty(const char* name, const Object& obj, bool ref = true) const {
     stream_->PrintProperty(name, obj, ref);
   }