Support being called as absolute path.

pkg/front_end/tool/fasta

 #!/bin/bash
 # Copyright (c) 2017, the Dart project authors.  Please see the AUTHORS file
 # 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.
 
 set -e
 
-REPO_DIR="$(git rev-parse --show-toplevel)"
+REPO_DIR="$(cd ${BASH_SOURCE%/*} && git rev-parse --show-toplevel)"
 
 # TODO(ahe): We currently rely on the Dart VM wrapper script to define
 # DART_CONFIGURATION for locating platform.dill.
 DART_VM="${REPO_DIR}/sdk/bin/dart"
 
 TOOL_DIR="${REPO_DIR}/pkg/front_end/tool/_fasta"
 
 KERNEL_BIN="${REPO_DIR}/pkg/kernel/bin"
 
 function stop {
@@ skipping 45 matching lines @@
     ;;
   generate-messages) SCRIPT="${TOOL_DIR}/generate_messages.dart";;
   *)
     stop "'$1' isn't a valid subcommand."
     ;;
 esac
 
 shift
 
 exec "${DART_VM}" -c "${SCRIPT}" "$@"