Added string appending to TextBuffer, removed STL use in vm_stats_impl.

runtime/platform/json.cc

Index: runtime/platform/json.cc
===================================================================
--- runtime/platform/json.cc	(revision 20477)
+++ runtime/platform/json.cc	(working copy)
@@ -545,6 +545,19 @@
 }
 
 
+void TextBuffer::AddString(const char* s) {
+  Printf("%s", s);
+}
+
+
+void TextBuffer::AddEscapedString(const char* s) {
+  intptr_t len = strlen(s);
+  for (int i = 0; i < len; i++) {
+    AddEscapedChar(s[i]);
+  }
+}
+
+
 void TextBuffer::EnsureCapacity(intptr_t len) {
   intptr_t remaining = buf_size_ - msg_len_;
   if (remaining <= len) {