Make stdin blocking when using Stdin::readByte and readLine.

runtime/bin/stdin_macos.cc

Index: runtime/bin/stdin_macos.cc
diff --git a/runtime/bin/stdin_macos.cc b/runtime/bin/stdin_macos.cc
index cb37a8b78ce12485360e697a5668435fb9591827..0bea869609749e059a7a639eae818fcdab9d67f7 100644
--- a/runtime/bin/stdin_macos.cc
+++ b/runtime/bin/stdin_macos.cc
@@ -8,16 +8,19 @@
 #include <termios.h>  // NOLINT
 
 #include "bin/stdin.h"
+#include "bin/fdutils.h"
 
 
 namespace dart {
 namespace bin {
 
 int Stdin::ReadByte() {
+  FDUtils::SetBlocking(fileno(stdin));
   int c = getchar();
   if (c == EOF) {
     c = -1;
   }
+  FDUtils::SetNonBlocking(fileno(stdin));
   return c;
 }