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

runtime/vm/snapshot.cc

Index: runtime/vm/snapshot.cc
diff --git a/runtime/vm/snapshot.cc b/runtime/vm/snapshot.cc
index b336512c34e069296cf0a687f84b6510a9bf6772..537bfcd3f178254ed073b5b0ace9d183b9fedef5 100644
--- a/runtime/vm/snapshot.cc
+++ b/runtime/vm/snapshot.cc
@@ -2515,13 +2515,11 @@ RawFunction* SnapshotWriter::IsSerializableClosure(RawClass* cls,
     ASSERT(!errorFunc.IsNull());
 
     // All other closures are errors.
-    const char* format = "Illegal argument in isolate message"
-        " : (object is a closure - %s %s)";
     UnmarkAll();  // Unmark objects now as we are about to print stuff.
-    intptr_t len = OS::SNPrint(NULL, 0, format,
-                               clazz.ToCString(), errorFunc.ToCString()) + 1;
-    char* chars = thread()->zone()->Alloc<char>(len);
-    OS::SNPrint(chars, len, format, clazz.ToCString(), errorFunc.ToCString());
+    char* chars = NULL;
+    SNPRINT(chars, thread()->zone()->Alloc<char>,
+        "Illegal argument in isolate message : (object is a closure - %s %s)",
+        clazz.ToCString(), errorFunc.ToCString());
     SetWriteException(Exceptions::kArgument, chars);
   }
   return Function::null();
@@ -2544,13 +2542,13 @@ void SnapshotWriter::CheckForNativeFields(RawClass* cls) {
   if (cls->ptr()->num_native_fields_ != 0) {
     // We do not allow objects with native fields in an isolate message.
     HANDLESCOPE(thread());
-    const char* format = "Illegal argument in isolate message"
-                         " : (object extends NativeWrapper - %s)";
     UnmarkAll();  // Unmark objects now as we are about to print stuff.
     const Class& clazz = Class::Handle(isolate(), cls);
-    intptr_t len = OS::SNPrint(NULL, 0, format, clazz.ToCString()) + 1;
-    char* chars = thread()->zone()->Alloc<char>(len);
-    OS::SNPrint(chars, len, format, clazz.ToCString());
+    char* chars = NULL;
+    SNPRINT(chars, thread()->zone()->Alloc<char>,
+            "Illegal argument in isolate message"
+            " : (object extends NativeWrapper - %s)",
+            clazz.ToCString());
     SetWriteException(Exceptions::kArgument, chars);
   }
 }