FFI varargs sample is brittle

[timsneath]

If varargs aren't supported in Dart FFI right now, should we be advertising them in this sample?

samples/ffi/primitives/primitives.dart

Lines 30 to 36 in 0b3835d

	// C multi sum function - int multi_sum(int nr_count, ...);
	//
	// Example of how to call C functions with varargs with a fixed arg count in
	// Dart
	typedef MultiSumFunc = Int32 Function(
	Int32 numCount, Int32 a, Int32 b, Int32 c);
	typedef MultiSum = int Function(int numCount, int a, int b, int c);

Context:
dart-lang/sdk#49460 (comment)
dart-lang/sdk#38578 (comment)

/cc @dcharkes