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..1d21afb037c3e8cbfef094c848869545039392fc 100755
--- a/sdk/bin/dartdoc
+++ b/sdk/bin/dartdoc
@@ -4,9 +4,10 @@
 # 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)"
+# dart-sdk/bin has been symlinked to. On MacOS, readlink -f doesn't work with
+# this case.
+SCRIPT=`readlink "$0" || echo "$0"`
+BIN_DIR="$(cd "${SCRIPT%/*}" ; pwd -P)"
 
 unset COLORS
 if test -t 1; then