Make File.create() fail when a directory exists at the same path

runtime/bin/file_macos.cc

Index: runtime/bin/file_macos.cc
diff --git a/runtime/bin/file_macos.cc b/runtime/bin/file_macos.cc
index 2b32bc1899fc263d5a36b3b8ee75a76c652ecd7e..d1259f757fc3d875b80339b98434b06dca10553b 100644
--- a/runtime/bin/file_macos.cc
+++ b/runtime/bin/file_macos.cc
@@ -230,7 +230,8 @@ File* File::OpenStdio(int fd) {
 bool File::Exists(const char* name) {
   struct stat st;
   if (NO_RETRY_EXPECTED(stat(name, &st)) == 0) {
-    return S_ISREG(st.st_mode);
+    // Everything but a directory and a link is a file to Dart.
+    return !S_ISDIR(st.st_mode) && !S_ISLNK(st.st_mode);
   } else {
     return false;
   }
@@ -242,7 +243,22 @@ bool File::Create(const char* name) {
   if (fd < 0) {
     return false;
   }
-  return (close(fd) == 0);
+  // File.create returns a File, so we shouldn't be giving the illusion that the
+  // call has created a file or that a file already exists if there is already
+  // an entity at the same path that is a directory or a link.
+  bool is_file = true;
+  struct stat st;
+  if (NO_RETRY_EXPECTED(fstat(fd, &st)) == 0) {
+    if (S_ISDIR(st.st_mode)) {
+      errno = EISDIR;
+      is_file = false;
+    } else if (S_ISLNK(st.st_mode)) {
+      errno = ENOENT;
+      is_file = false;
+    }
+  }
+  FDUtils::SaveErrorAndClose(fd);
+  return is_file;
 }