Update the way that pub determines DART_SDK on macos/linux.

utils/pub/sdk/pub

Index: utils/pub/sdk/pub
===================================================================
--- utils/pub/sdk/pub	(revision 13027)
+++ utils/pub/sdk/pub	(working copy)
@@ -1,7 +1,11 @@
-#!/bin/sh
+#!/bin/bash

[Bob Nystrom, 2012/09/28 17:38:04]

I'm not a Unix expert. Will this cause problems for users that don't typically
use (or have?) bash as their shell?

[dgrove, 2012/09/28 17:49:29]

This only causes issues for people that don't have bash installed at all. 

Bash is available by default on both linux and macos. This just means that the
script will be executed by bash - it shouldn't cause problems for users that are
using other shells.

 # Run pub.dart on the Dart VM. This script assumes the Dart SDK's directory
 # structure.
 
-BIN_DIR=`cd "\$(dirname "\$0")"; pwd -P`
-export DART_SDK=`dirname $BIN_DIR`
-$DART_SDK/bin/dart $DART_SDK/util/pub/pub.dart $@
+# Setting BIN_DIR this way is ugly, but is needed to handle the case where
+# dart-sdk/bin has been symlinked to. On MacOS, readlink doesn't work
+# with this case.
+BIN_DIR="$(cd "${0%/*}" ; pwd -P)"
+export DART_SDK="$(cd "${BIN_DIR%/*}" ; pwd -P)"
+
+exec "$BIN_DIR"/dart "$DART_SDK"/util/pub/pub.dart $@