language/node: fix prepublish scripts

This commit adds support for installing node modules with language node,
which are executing one of their dev(dependencies) in their prepublish
script.

Refs: Homebrew/homebrew-core#14827

Library/Homebrew/language/node.rb

@@ -4,15 +4,23 @@ def self.npm_cache_config
       "cache=#{HOMEBREW_CACHE}/npm_cache\n"
     end
 
-    def self.pack_for_installation
+    def self.pack_for_installation(prepublish_requires_deps)
+      # Some packages are requiring (dev)dependencies to be in place when
+      # running the prepublish script (which is run before pack). In this case
+      # we have to install all dependencies a first time already before
+      # executing npm pack (and a second time when doing the actual install).
+      safe_system "npm", "install" if prepublish_requires_deps
+
       # Homebrew assumes the buildpath/testpath will always be disposable
       # and from npm 5.0.0 the logic changed so that when a directory is
       # fed to `npm install` only symlinks are created linking back to that
       # directory, consequently breaking that assumption. We require a tarball
       # because npm install creates a "real" installation when fed a tarball.
-      output = Utils.popen_read("npm pack").chomp
-      raise "npm failed to pack #{Dir.pwd}" unless $CHILD_STATUS.exitstatus.zero?
-      output
+      output = Utils.popen_read("npm pack")
+      unless $CHILD_STATUS.exitstatus.zero? && !output.lines.empty?
+        raise "npm failed to pack #{Dir.pwd}"
+      end
+      output.lines.last.chomp
     end
 
     def self.setup_npm_environment
@@ -32,13 +40,13 @@ def self.setup_npm_environment
       end
     end
 
-    def self.std_npm_install_args(libexec)
+    def self.std_npm_install_args(libexec, prepublish_requires_deps = false)
       setup_npm_environment
       # tell npm to not install .brew_home by adding it to the .npmignore file
       # (or creating a new one if no .npmignore file already exists)
       open(".npmignore", "a") { |f| f.write("\n.brew_home\n") }
 
-      pack = pack_for_installation
+      pack = pack_for_installation(prepublish_requires_deps)
 
       # npm install args for global style module format installed into libexec
       %W[

docs/Node-for-Formula-Authors.md

@@ -72,6 +72,12 @@ This will install your Node module in npm's global module style with a custom pr
 bin.install_symlink Dir["#{libexec}/bin/*"]
 ```
 
+*Note:* If your module requires a (dev)dependencies to to present at npm prepublish time, you have to invoke `std_npm_install_args` with `prepublish_requires_deps` set to `true` like:
+
+```ruby
+system "npm", "install", *Language::Node.std_npm_install_args(libexec, prepublish_requires_deps = true)
+```
+
 ### Installing module dependencies locally with `local_npm_install_args`
 
 In your formula's `install` method, do any installation steps which need to be done before the `npm install` step and then `cd` to the top level of the included Node module. Then, use `system` with `Language::Node.local_npm_install_args` to invoke `npm install` like: