Polymer: make reproduce.sh check for tools it depends on

third_party/polymer/v1_0/reproduce.sh

 #!/bin/bash
 
 # Copyright 2014 The Chromium Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
 # Reproduces the content of 'components' and 'components-chromium' using the
 # list of dependencies from 'bower.json'. Downloads needed packages and makes
 # Chromium specific modifications. To launch the script you need 'bower' and
 # 'crisper' installed on your system.
 
+check_dep() {
+  eval "$1" >/dev/null 2>&1

[michaelpg, 2016/02/17 05:14:29]

why: eval "$1"
why not: which "$2"

[Dan Beam, 2016/02/17 17:22:31]

https://codereview.chromium.org/1702553002/diff/160001/third_party/polymer/v1...

as you found

+  if [ $? -ne 0 ]; then
+    echo >&2 "This script requires $2."
+    echo >&2 "Have you tried $3?"
+    exit 1
+  fi
+}
+
+check_dep "which npm" "npm" "visiting https://nodejs.org/en/"
+check_dep "which bower" "bower" "npm install -g bower"
+check_dep "which crisper" "crisper" "npm install -g crisper"
+check_dep "which rsync" "rsync" "apt-get install rsync"
+
 set -e
 
 cd "$(dirname "$0")"
 
 rm -rf components
 rm -rf ../../web-animations-js/sources
 
 bower install --no-color
 
 rm components/*/.travis.yml
@@ skipping 54 matching lines @@
   echo
   echo 'These files appear to have been removed:'
   echo "${deleted}" | sed 's/^/  /'
 fi
 
 if [[ ! -z "${new}${deleted}" ]]; then
   echo
 fi
 
 python create_components_summary.py > components_summary.txt