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..e26f565e46ce79274aa81ff34527176fd011371e 100644
--- a/runtime/vm/os.h
+++ b/runtime/vm/os.h
@@ -7,6 +7,37 @@
 
 #include "vm/globals.h"
 
+// SNPRINT is a macro wrapping OS::SNPrint which makes two calls. The first call
+// is with NULL for the buffer. This call returns the number of bytes that would
+// have been written had the buffer not been NULL. The macro allocates that many
+// bytes plus room for the null terminator, and makes a second call to
+// OS::SNPrint, passing the allocated buffer.
+//
+// The reason for wrapping OS::SNPrint in a macro is so that the format string
+// literal can be passed directly to both calls without doing a copy-paste.
+// The format string literal must be passed directly to both calls to make sure
+// that we get compiler errors when the format string does not match the passed
+// arguments.
+#if defined(_MSC_VER_)
+#define SNPRINT(buffer, allocator, format, ...)                                \
+  {                                                                            \
+    intptr_t len = OS::SNPrint(NULL, 0, (format), __VA_ARGS__);                \
+    (buffer) = reinterpret_cast<char*>((allocator)(len + 1));                  \
+    ASSERT(buffer != NULL);                                                    \
+    OS::SNPrint((buffer), len + 1, (format), __VA_ARGS__);                     \
+  }                                                                            \
+
+#else
+#define SNPRINT(buffer, allocator, format, ...)                                \
+  {                                                                            \
+    intptr_t len = OS::SNPrint(NULL, 0, (format), ##__VA_ARGS__);              \
+    (buffer) = reinterpret_cast<char*>((allocator)(len + 1));                  \
+    ASSERT(buffer != NULL);                                                    \
+    OS::SNPrint((buffer), len + 1, (format), ##__VA_ARGS__);                   \
+  }                                                                            \
+
+#endif
+
 // Forward declarations.
 struct tm;