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

sdk/bin/pub

Index: sdk/bin/pub
diff --git a/sdk/bin/pub b/sdk/bin/pub
index 231da3ebc4cbc2580e647e355e622608a6fa7ab5..42f84fce029f3bf86b7b05916877c25dc8158b27 100755
--- a/sdk/bin/pub
+++ b/sdk/bin/pub
@@ -6,10 +6,21 @@
 # Run pub.dart on the Dart VM. This script assumes the Dart SDK's directory
 # structure.
 
-# 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 "${PROG_NAME%/*}" ; pwd -P)"
+
 SDK_DIR="$(cd "${BIN_DIR}/.." ; pwd -P)"
 
 SNAPSHOT="$BIN_DIR/snapshots/pub.dart.snapshot"
@@ -40,4 +51,4 @@ else
   PUB="$SDK_DIR/lib/_internal/pub/bin/pub.dart"
 
   exec "$DART" "--package-root=$PACKAGES_DIR" "$PUB" "$@"
-fi
+fi