Make it possible to symlink to dartdoc and pub in the SDK.

sdk/bin/dartdoc

 #!/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 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 "$(cd "$(dirname $file)"; pwd -P)/$(basename "$file")"
+}
+
+# Unlike $0, $BASH_SOURCE points to the absolute path of this file. Follow
+# symlinks to handle the case where dart-sdk/bin or dart-sdk/bin/pub have been
+# linked to.
+BIN_DIR="$(dirname "$(follow_links "$BASH_SOURCE")")"
 
 unset COLORS
 if test -t 1; then
   # Stdout is a terminal.
   if test 8 -le `tput colors`; then
     # Stdout has at least 8 colors, so enable colors.
     COLORS="--enable-diagnostic-colors"
   fi
 fi
 
 unset SNAPSHOT
 if test -f "$BIN_DIR/../lib/_internal/dartdoc/bin/dartdoc.dart.snapshot"; then
   # TODO(ahe): Remove the following line when we are relatively sure it works.
   echo Using snapshot "$BIN_DIR/../lib/_internal/dartdoc/bin/dartdoc.dart.snapshot" 1>&2
   SNAPSHOT="--use-script-snapshot=$BIN_DIR/../lib/_internal/dartdoc/bin/dartdoc.dart.snapshot"
 fi
 exec "$BIN_DIR"/dart --heap_growth_rate=32 "--package-root=$BIN_DIR/../packages/" $SNAPSHOT "$BIN_DIR/../lib/_internal/dartdoc/bin/dartdoc.dart" "--package-root=$BIN_DIR/../packages/" $COLORS "$@"