Make bower an optional dependency

Closes [#531].

According to `bower` warning messages, the project is semi-deprecated:

```
While Bower is maintained, we recommend Yarn and Webpack for *new*
front-end projects! Yarn's advantage is security and reliability, and
Webpack's is support for both CommonJS and AMD projects. Currently
there's no migration path but we hope you'll help us figure out one.
```

As such, the build process will skip `bower install` and won't require a
`bower` if the application's `bower.json` file is missing.

[#531]: #531

CHANGELOG.md

@@ -1,6 +1,10 @@
 master
 ------
 
+* Don't require `bower` installation if `bower.json` is missing [#532]
+
+[#532]: https://github.com/thoughtbot/ember-cli-rails/pull/532
+
 0.8.5
 -----
 

README.md

@@ -270,7 +270,8 @@ To configure your EmberCLI-Rails applications for Heroku:
 1. Execute `rails generate ember:heroku`.
 1. Commit the newly generated files.
 1. [Add the NodeJS buildpack][buildpack] and configure NPM to include the
-   `bower` dependency's executable file.
+   `bower` dependency's executable file (if your build process requires
+   `bower`).
 
 ```sh
 $ heroku buildpacks:clear
@@ -328,11 +329,16 @@ contains the directory or directories that contain the `bower` and `npm`
 executables.
 
 #### For faster deployments
-Place the following in your deploy/<environment>.rb
+
+Place the following in your `deploy/<environment>.rb`
+
 ```ruby
 set :linked_dirs, %w{<ember-app-name>/node_modules <ember-app-name>/bower_components}
 ```
-to avoid rebuilding all the node modules and bower components with every deploy. Replace `<ember-app-name>` with the name of your ember app (default is `frontend`).
+
+to avoid rebuilding all the node modules and bower components with every deploy.
+Replace `<ember-app-name>` with the name of your ember app (default is
+`frontend`).
 
 ## Override
 

lib/ember_cli/path_set.rb

@@ -35,6 +35,10 @@ def gemfile
       @gemfile ||= root.join("Gemfile")
     end
 
+    def bower_json
+      ember_cli_root.join("bower.json")
+    end
+
     def ember
       @ember ||= begin
         root.join("node_modules", "ember-cli", "bin", "ember").tap do |path|
@@ -64,7 +68,7 @@ def build_error_file
     def bower
       @bower ||= begin
         path_for_executable("bower").tap do |bower_path|
-          if bower_path.blank? || !bower_path.executable?
+          if bower_json.exist? && (bower_path.blank? || !bower_path.executable?)
             fail DependencyError.new <<-MSG.strip_heredoc
                 Bower is required by EmberCLI
 

lib/ember_cli/shell.rb

@@ -41,13 +41,15 @@ def install
         clean_ember_dependencies!
       end
 
-      if paths.yarn
+      if paths.yarn.present? && Pathname.new(paths.yarn).executable?
         run! "#{paths.yarn} install"
       else
         run! "#{paths.npm} prune && #{paths.npm} install"
       end
 
-      run! "[ -f bower.json ] && #{paths.bower} prune && #{paths.bower} install"
+      if paths.bower_json.exist?
+        run! "#{paths.bower} prune && #{paths.bower} install"
+      end
     end
 
     def test

spec/lib/ember_cli/path_set_spec.rb

@@ -119,14 +119,38 @@
     end
 
     context "when it is missing from the $PATH" do
-      it "raises a helpful exception" do
-        stub_which(bower: nil)
+      context "bower.json exists" do
+        it "raises a helpful exception" do
+          create_file(ember_cli_root.join("bower.json"))
+          stub_which(bower: nil)
+          path_set = build_path_set
+
+          expect { path_set.bower }.
+            to raise_error(EmberCli::DependencyError, /bower is required/i)
+        end
+
+        context "bower.json is missing" do
+          it "returns nil" do
+            stub_which(bower: nil)
+            path_set = build_path_set
+
+            bower = path_set.bower
+
+            expect(bower).to be_nil
+          end
+        end
+      end
+    end
+  end
 
-        path_set = build_path_set
+  describe "#bower_json" do
+    it "is a child of #root" do
+      app = build_app(name: "foo")
+      path_set = build_path_set(app: app)
 
-        expect { path_set.bower }.
-          to raise_error(EmberCli::DependencyError, /bower is required/i)
-      end
+      bower_json = path_set.bower_json
+
+      expect(bower_json).to eq ember_cli_root.join("bower.json")
     end
   end
 
@@ -164,12 +188,6 @@
   end
 
   describe "#yarn" do
-    it "is not enabled by default" do
-      path_set = build_path_set
-
-      expect(path_set.yarn).to be nil
-    end
-
     it "can be overridden" do
       fake_yarn = create_executable(ember_cli_root.join("yarn"))
       app = build_app(options: { yarn_path: fake_yarn.to_s })
@@ -185,11 +203,23 @@
       stub_which(yarn: fake_yarn.to_s)
       app = build_app(options: { yarn: true })
       path_set = build_path_set(app: app)
+      create_executable(fake_yarn)
 
       yarn = path_set.yarn
 
       expect(yarn).to eq(fake_yarn).and(be_executable)
     end
+
+    context "when the executable isn't installed on the system" do
+      it "returns nil" do
+        stub_which(yarn: nil)
+        path_set = build_path_set
+
+        yarn = path_set.yarn
+
+        expect(yarn).to be_nil
+      end
+    end
   end
 
   describe "#node_modules" do