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

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 "$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)"
 
 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
 
 SNAPSHOT="$BIN_DIR/snapshots/utils_wrapper.dart.snapshot"
 
 if test -f $SNAPSHOT; then
   # TODO(ahe): Remove the following line when we are relatively sure it works.
   echo Using snapshot $SNAPSHOT 1>&2
   exec "$BIN_DIR"/dart --heap_growth_rate=32 \
       "--package-root=$BIN_DIR/../packages/" $SNAPSHOT dartdoc $COLORS \
       "--package-root=$BIN_DIR/../packages/" "--library-root=$BIN_DIR/.." "$@"
 else
   exec "$BIN_DIR"/dart --heap_growth_rate=32 \
       "--package-root=$BIN_DIR/../packages/" \
       "$BIN_DIR/../lib/_internal/dartdoc/bin/dartdoc.dart" $COLORS "$@"
 fi