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

sdk/bin/dartdoc

Index: sdk/bin/dartdoc
diff --git a/sdk/bin/dartdoc b/sdk/bin/dartdoc
index 3f223678f655240fa886e78badda0250211bdfb8..bdb7b953c2ff1136956362a6a852f761efcac256 100755
--- a/sdk/bin/dartdoc
+++ b/sdk/bin/dartdoc
@@ -3,10 +3,19 @@
 # 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