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

sdk/bin/pub

 #!/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.
 
 # 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)"
+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 "$BASH_SOURCE")"
+
+# Handle the case where dart-sdk/bin has been symlinked to.
+BIN_DIR="$(cd "${PROG_NAME%/*}" ; pwd -P)"
+
 SDK_DIR="$(cd "${BIN_DIR}/.." ; pwd -P)"
 
 SNAPSHOT="$BIN_DIR/snapshots/pub.dart.snapshot"
 
 if test -f "$SNAPSHOT"; then
   # We are running the snapshot in the built SDK.
   DART="$BIN_DIR/dart"
   exec "$DART" "$SNAPSHOT" "$@"
 else
   # We are running pub from source in the development repo.
@@ skipping 10 matching lines @@
   fi
 
   # Use the Dart binary in the built SDK so pub can find the version file next
   # to it.
   DART="$BUILD_DIR/dart-sdk/bin/dart"
   PACKAGES_DIR="$BUILD_DIR/packages/"
 
   PUB="$SDK_DIR/lib/_internal/pub/bin/pub.dart"
 
   exec "$DART" "--package-root=$PACKAGES_DIR" "$PUB" "$@"
-fi
+fi