Support symlinking to dart-sdk executables on Linux and Mac.

sdk/bin/dart

 #!/bin/bash
 # Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
 # for details. All rights reserved. Use of this source code is governed by a
 # BSD-style license that can be found in the LICENSE file.
 
-# Setting CUR_DIR this way is ugly, but is needed to handle the case where
-# sdk/bin has been symlinked to. On MacOS, readlink doesn't work with this case.
-CUR_DIR="$(cd "${0%/*}" ; pwd -P)"
+function follow_links() {
+  file="$1"
+  while [ -h "$file" ]; do
+    # On Mac OS, readlink -f doesn't work.
+    file="$(readlink "$file")"
+  done
+  echo "$file"
+}
+
+# Unlike $0, $BASH_SOURCE points to the absolute path of this file.
+PROG_NAME="$(follow_links "$BASH_SOURCE")"
+
+# Handle the case where dart-sdk/bin has been symlinked to.
+CUR_DIR="$(cd "${PROG_NAME%/*}" ; pwd -P)"
 
 if [ -z "$DART_CONFIGURATION" ];
 then
   DART_CONFIGURATION="ReleaseIA32"
 fi
 
-if [[ `uname` == 'Darwin' ]]; 
+if [[ `uname` == 'Darwin' ]];
 then
   BIN_DIR="$CUR_DIR"/../../xcodebuild/$DART_CONFIGURATION
 else
   BIN_DIR="$CUR_DIR"/../../out/$DART_CONFIGURATION
-fi 
+fi
 
 exec "$BIN_DIR"/dart "$@"