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

sdk/bin/dartdoc

Index: sdk/bin/dartdoc
diff --git a/sdk/bin/dartdoc b/sdk/bin/dartdoc
index f95cb169787e281742dd27c579132710aeb99cb4..45ba5125b3ad43f0a18139b0febcf246050a05e1 100755
--- a/sdk/bin/dartdoc
+++ b/sdk/bin/dartdoc
@@ -3,10 +3,20 @@
 # 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