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

runtime/vm/os.h

Index: runtime/vm/os.h
diff --git a/runtime/vm/os.h b/runtime/vm/os.h
index 60456e4200add3ae15ecf75667a1e2ab1d291b5a..8b60df4eaf7cb85adcbb5829fe7129bfb10cd120 100644
--- a/runtime/vm/os.h
+++ b/runtime/vm/os.h
@@ -7,6 +7,7 @@
 
 #include "vm/globals.h"
 
+
 // Forward declarations.
 struct tm;
 
@@ -14,6 +15,7 @@ namespace dart {
 
 // Forward declarations.
 class Isolate;
+class Zone;
 
 // Interface to the underlying OS platform.
 class OS {
@@ -120,6 +122,12 @@ class OS {
                       const char* format,
                       va_list args);
 
+  // Allocate a string and print formatted output into the buffer.
+  // Uses the zone for allocation if one if provided, and otherwise uses
+  // malloc.
+  static char* SNCreate(Zone* zone, const char* format, ...)
+      PRINTF_ATTRIBUTE(2, 3);
+
   // Converts a C string which represents a valid dart integer into a 64 bit
   // value.
   // Returns false if it is unable to convert the string to a 64 bit value,