Uses SNPRINT macro where possible. Otherwise uses #define for format.

runtime/vm/os_macos.cc

Index: runtime/vm/os_macos.cc
diff --git a/runtime/vm/os_macos.cc b/runtime/vm/os_macos.cc
index efdcd2bbf937c479c903ddb901bce0a19a0f7592..febdab329cb4c305b92378403cf94ba7d7d85ea7 100644
--- a/runtime/vm/os_macos.cc
+++ b/runtime/vm/os_macos.cc
@@ -205,6 +205,34 @@ int OS::SNPrint(char* str, size_t size, const char* format, ...) {
 }
 
 
+char* OS::SNCreate(Zone* zone, const char* format, ...) {
+  va_list args;
+  va_start(args, format);
+
+  // Measure.
+  va_list measure_args;
+  va_copy(measure_args, args);
+  intptr_t len = VSNPrint(NULL, 0, format, measure_args);
+  va_end(measure_args);
+
+  char* buffer;
+  if (zone) {
+    buffer = zone->Alloc<char>(len + 1);
+  } else {
+    buffer = reinterpret_cast<char*>(malloc(len + 1));
+  }
+  ASSERT(buffer != NULL);
+
+  // Print.
+  va_list print_args;
+  va_copy(print_args, args);
+  VSNPrint(buffer, len + 1, format, print_args);
+  va_end(print_args);
+  va_end(args);
+  return buffer;
+}
+
+
 int OS::VSNPrint(char* str, size_t size, const char* format, va_list args) {
   int retval = vsnprintf(str, size, format, args);
   if (retval < 0) {