Herokuへデプロイ時のエラーが解決できない。Precompiling assets failed

前提・実現したいこと

Herokuへデプロイしたいです。

Ruby on Railsでアプリケーションを作り、Herokuへデプロイしたいのですが、エラーが出て、数時間悩まされ続けています。

発生している問題・エラーメッセージ

Precompiling assets failed.が赤く表示されています。

該当のソースコード

以下、git push heroku masterした後のターミナルです。
一部文字数のため、fetch成功のところを省略しています。

git
1
2remote:        Bundle complete! 27 Gemfile dependencies, 73 gems now installed.
3remote:        Gems in the groups development and test were not installed.
4remote:        Bundled gems are installed into ./vendor/bundle.
5remote:        Post-install message from sass:
6remote:        
7remote:        Ruby Sass is deprecated and will be unmaintained as of 26 March 2019.
8remote:        
9remote:        * If you use Sass as a command-line tool, we recommend using Dart Sass, the new
10remote:          primary implementation: https://sass-lang.com/install
11remote:        
12remote:        * If you use Sass as a plug-in for a Ruby web framework, we recommend using the
13remote:          sassc gem: https://github.com/sass/sassc-ruby#readme
14remote:        
15remote:        * For more details, please refer to the Sass blog:
16remote:          http://sass.logdown.com/posts/7081811
17remote:        
18remote:        Bundle completed (52.01s)
19remote:        Cleaning up the bundler cache.
20remote:        Warning: the running version of Bundler (1.15.2) is older than the version that created the lockfile (1.16.3). We suggest you upgrade to the latest version of Bundler by running `gem install bundler`.
21remote:        The latest bundler is 1.16.5, but you are currently running 1.15.2.
22remote:        To update, run `gem install bundler`
23remote: -----> Installing node-v8.10.0-linux-x64
24remote: -----> Detecting rake tasks
25remote: -----> Preparing app for Rails asset pipeline
26remote:        Running: rake assets:precompile
27remote:        Yarn executable was not detected in the system.
28remote:        Download Yarn at https://yarnpkg.com/en/docs/install
29remote:        rake aborted!
30remote:        Uglifier::Error: Unexpected token: keyword (const). To use ES6 syntax, harmony mode must be enabled with Uglifier.new(:harmony => true).
31remote:        /tmp/build_bc2e4864ab5a1aeebc622dd549e5990e/vendor/bundle/ruby/2.5.0/gems/uglifier-4.1.18/lib/uglifier.rb:234:in `parse_result'
32remote:        /tmp/build_bc2e4864ab5a1aeebc622dd549e5990e/vendor/bundle/ruby/2.5.0/gems/uglifier-4.1.18/lib/uglifier.rb:216:in `run_uglifyjs'
33remote:        /tmp/build_bc2e4864ab5a1aeebc622dd549e5990e/vendor/bundle/ruby/2.5.0/gems/uglifier-4.1.18/lib/uglifier.rb:168:in `compile'
34remote:        /tmp/build_bc2e4864ab5a1aeebc622dd549e5990e/vendor/bundle/ruby/2.5.0/gems/sprockets-3.7.2/lib/sprockets/uglifier_compressor.rb:53:in `call'
35remote:        /tmp/build_bc2e4864ab5a1aeebc622dd549e5990e/vendor/bundle/ruby/2.5.0/gems/sprockets-3.7.2/lib/sprockets/uglifier_compressor.rb:28:in `call'
36remote:        /tmp/build_bc2e4864ab5a1aeebc622dd549e5990e/vendor/bundle/ruby/2.5.0/gems/sprockets-3.7.2/lib/sprockets/processor_utils.rb:75:in `call_processor'
37remote:        /tmp/build_bc2e4864ab5a1aeebc622dd549e5990e/vendor/bundle/ruby/2.5.0/gems/sprockets-3.7.2/lib/sprockets/processor_utils.rb:57:in `block in call_processors'
38remote:        /tmp/build_bc2e4864ab5a1aeebc622dd549e5990e/vendor/bundle/ruby/2.5.0/gems/sprockets-3.7.2/lib/sprockets/processor_utils.rb:56:in `reverse_each'
39remote:        /tmp/build_bc2e4864ab5a1aeebc622dd549e5990e/vendor/bundle/ruby/2.5.0/gems/sprockets-3.7.2/lib/sprockets/processor_utils.rb:56:in `call_processors'
40remote:        /tmp/build_bc2e4864ab5a1aeebc622dd549e5990e/vendor/bundle/ruby/2.5.0/gems/sprockets-3.7.2/lib/sprockets/loader.rb:134:in `load_from_unloaded'
41remote:        /tmp/build_bc2e4864ab5a1aeebc622dd549e5990e/vendor/bundle/ruby/2.5.0/gems/sprockets-3.7.2/lib/sprockets/loader.rb:60:in `block in load'
42remote:        /tmp/build_bc2e4864ab5a1aeebc622dd549e5990e/vendor/bundle/ruby/2.5.0/gems/sprockets-3.7.2/lib/sprockets/loader.rb:317:in `fetch_asset_from_dependency_cache'
43remote:        /tmp/build_bc2e4864ab5a1aeebc622dd549e5990e/vendor/bundle/ruby/2.5.0/gems/sprockets-3.7.2/lib/sprockets/loader.rb:44:in `load'
44remote:        /tmp/build_bc2e4864ab5a1aeebc622dd549e5990e/vendor/bundle/ruby/2.5.0/gems/sprockets-3.7.2/lib/sprockets/cached_environment.rb:20:in `block in initialize'
45remote:        /tmp/build_bc2e4864ab5a1aeebc622dd549e5990e/vendor/bundle/ruby/2.5.0/gems/sprockets-3.7.2/lib/sprockets/cached_environment.rb:47:in `load'
46remote:        /tmp/build_bc2e4864ab5a1aeebc622dd549e5990e/vendor/bundle/ruby/2.5.0/gems/sprockets-3.7.2/lib/sprockets/base.rb:66:in `find_asset'
47remote:        /tmp/build_bc2e4864ab5a1aeebc622dd549e5990e/vendor/bundle/ruby/2.5.0/gems/sprockets-3.7.2/lib/sprockets/base.rb:73:in `find_all_linked_assets'
48remote:        /tmp/build_bc2e4864ab5a1aeebc622dd549e5990e/vendor/bundle/ruby/2.5.0/gems/sprockets-3.7.2/lib/sprockets/manifest.rb:142:in `block in find'
49remote:        /tmp/build_bc2e4864ab5a1aeebc622dd549e5990e/vendor/bundle/ruby/2.5.0/gems/sprockets-3.7.2/lib/sprockets/legacy.rb:114:in `block (2 levels) in logical_paths'
50remote:        /tmp/build_bc2e4864ab5a1aeebc622dd549e5990e/vendor/bundle/ruby/2.5.0/gems/sprockets-3.7.2/lib/sprockets/path_utils.rb:228:in `block in stat_tree'
51remote:        /tmp/build_bc2e4864ab5a1aeebc622dd549e5990e/vendor/bundle/ruby/2.5.0/gems/sprockets-3.7.2/lib/sprockets/path_utils.rb:212:in `block in stat_directory'
52remote:        /tmp/build_bc2e4864ab5a1aeebc622dd549e5990e/vendor/bundle/ruby/2.5.0/gems/sprockets-3.7.2/lib/sprockets/path_utils.rb:209:in `each'
53remote:        /tmp/build_bc2e4864ab5a1aeebc622dd549e5990e/vendor/bundle/ruby/2.5.0/gems/sprockets-3.7.2/lib/sprockets/path_utils.rb:209:in `stat_directory'
54remote:        /tmp/build_bc2e4864ab5a1aeebc622dd549e5990e/vendor/bundle/ruby/2.5.0/gems/sprockets-3.7.2/lib/sprockets/path_utils.rb:227:in `stat_tree'
55remote:        /tmp/build_bc2e4864ab5a1aeebc622dd549e5990e/vendor/bundle/ruby/2.5.0/gems/sprockets-3.7.2/lib/sprockets/legacy.rb:105:in `each'
56remote:        /tmp/build_bc2e4864ab5a1aeebc622dd549e5990e/vendor/bundle/ruby/2.5.0/gems/sprockets-3.7.2/lib/sprockets/legacy.rb:105:in `block in logical_paths'
57remote:        /tmp/build_bc2e4864ab5a1aeebc622dd549e5990e/vendor/bundle/ruby/2.5.0/gems/sprockets-3.7.2/lib/sprockets/legacy.rb:104:in `each'
58remote:        /tmp/build_bc2e4864ab5a1aeebc622dd549e5990e/vendor/bundle/ruby/2.5.0/gems/sprockets-3.7.2/lib/sprockets/legacy.rb:104:in `logical_paths'
59remote:        /tmp/build_bc2e4864ab5a1aeebc622dd549e5990e/vendor/bundle/ruby/2.5.0/gems/sprockets-3.7.2/lib/sprockets/manifest.rb:140:in `find'
60remote:        /tmp/build_bc2e4864ab5a1aeebc622dd549e5990e/vendor/bundle/ruby/2.5.0/gems/sprockets-3.7.2/lib/sprockets/manifest.rb:186:in `compile'
61remote:        /tmp/build_bc2e4864ab5a1aeebc622dd549e5990e/vendor/bundle/ruby/2.5.0/gems/sprockets-rails-3.2.1/lib/sprockets/rails/task.rb:68:in `block (3 levels) in define'
62remote:        /tmp/build_bc2e4864ab5a1aeebc622dd549e5990e/vendor/bundle/ruby/2.5.0/gems/sprockets-3.7.2/lib/rake/sprocketstask.rb:147:in `with_logger'
63remote:        /tmp/build_bc2e4864ab5a1aeebc622dd549e5990e/vendor/bundle/ruby/2.5.0/gems/sprockets-rails-3.2.1/lib/sprockets/rails/task.rb:67:in `block (2 levels) in define'
64remote:        /tmp/build_bc2e4864ab5a1aeebc622dd549e5990e/vendor/bundle/ruby/2.5.0/gems/rake-12.3.1/exe/rake:27:in `<top (required)>'
65remote:        Tasks: TOP => assets:precompile
66remote:        (See full trace by running task with --trace)
67remote: 
68remote:  !
69remote:  !     Precompiling assets failed.
70remote:  !
71remote:  !     Push rejected, failed to compile Ruby app.
72remote: 
73remote:  !     Push failed
74remote: Verifying deploy...
75remote: 
76remote: !	Push rejected to secure-citadel-87684.
77remote: 
78To https://git.heroku.com/secure-citadel-87684.git
79 ! [remote rejected] master -> master (pre-receive hook declined)
80error: failed to push some refs to 'https://git.heroku.com/secure-citadel-87684.git'
81