Fix for bug dart 3639: Process.run gets stuck on Mac

runtime/bin/platform_linux.cc

Index: runtime/bin/platform_linux.cc
diff --git a/runtime/bin/platform_linux.cc b/runtime/bin/platform_linux.cc
index 495644f385604603472dd1caf0b3a8ddc45b31c4..e9ca3779503f8760257fb2ed1fa69ec5c80c61b6 100644
--- a/runtime/bin/platform_linux.cc
+++ b/runtime/bin/platform_linux.cc
@@ -20,6 +20,14 @@ bool Platform::Initialize() {
     perror("Setting signal handler failed");
     return false;
   }
+  // Unblock SIGCHLD as waiting on spawned child process depends
+  // on successful interception of this signal.
+  sigset_t newset;
+  sigemptyset(&newset);
+  sigaddset(&newset, SIGCHLD);
+  if (sigprocmask(SIG_UNBLOCK, &newset, NULL) != 0) {
+    perror("Unblocking SIGCHLD signal failed");
+  }
   return true;
 }