Add support to trace warnings in TraceBuffer.

runtime/vm/json_stream.cc

Index: runtime/vm/json_stream.cc
===================================================================
--- runtime/vm/json_stream.cc	(revision 37297)
+++ runtime/vm/json_stream.cc	(working copy)
@@ -216,6 +216,12 @@
 }
 
 
+void JSONStream::PrintValueNoEscape(const char* s) {
+  PrintCommaIfNeeded();
+  buffer_.Printf("%s", s);
+}
+
+
 void JSONStream::PrintfValue(const char* format, ...) {
   PrintCommaIfNeeded();
 
@@ -289,6 +295,12 @@
 }
 
 
+void JSONStream::PrintPropertyNoEscape(const char* name, const char* s) {
+  PrintPropertyName(name);
+  PrintValueNoEscape(s);
+}
+
+
 void JSONStream::PrintProperty(const char* name, const DebuggerEvent* event) {
   PrintPropertyName(name);
   PrintValue(event);