Add testing subcommand to fasta tool.

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)"
 
 # TODO(ahe): We currently rely on the Dart VM wrapper script to define
@@ skipping 29 matching lines @@
   dump-ir)
     SCRIPT="${KERNEL_BIN}/dump.dart"
     if [ "$#" = "2" ]; then
       # Write to stdout if no target is given.
       set -- "$@" /dev/fd/1
     fi
     if [ "$#" != "3" ]; then
       stop "Usage: $1 dillfile [output]"
     fi
     ;;
+  testing)
+    SCRIPT="${REPO_DIR}/pkg/testing/bin/testing.dart"
+    set -- "$@" "--config=${REPO_DIR}/pkg/front_end/test/fasta/testing.json"
+    ;;
   *)
     stop "'$1' isn't a valid subcommand."
     ;;
 esac
 
 shift
 
-exec "${DART_VM}" -c "${SCRIPT}" "$@"
+DART_CONFIGURATION=${DART_CONFIGURATION:-ReleaseX64} exec "${DART_VM}" -c "${SCRIPT}" "$@"