Ensure proper path traversals for linked commands;

sdk/bin/pub

 #!/bin/bash
 # Run pub.dart on the Dart VM. This script assumes the Dart SDK's directory
 # structure.
 
 # Setting BIN_DIR this way is ugly, but is needed to handle the case where
 # dart-sdk/bin has been symlinked to. On MacOS, readlink doesn't work
 # with this case.
-BIN_DIR="$(cd "${0%/*}" ; pwd -P)"
-DART_SDK="$(cd "${BIN_DIR%/*}" ; pwd -P)"
+function link_dir() {
+  a="$1"
+  while [ -h "$a" ]; do
+    # On Mac OS, readlink -f doesn't work.
+    a="$(readlink "$a")"
+  done
+  # if dir linked, not file, need to follow dir path:
+  echo $(cd $(dirname $a); pwd -P)
+}
+
+BIN_DIR="$(link_dir "$0")"
+DART_SDK="${BIN_DIR%/*}"
 
 exec "$BIN_DIR"/dart "$DART_SDK"/bin/snapshots/pub.dart.snapshot $@