Fix `your_first_dart_app` branch to work

[nwtgck]

Hi, @eee-c.

Thank you for your great dart-comics!

Environment

$ dart --version
Dart VM version: 1.24.3 (Wed Dec 13 23:26:59 2017) on "macos_x64"

$ pub --version
Pub 1.24.3

Problem

I tried dart-comics on your_first_dart_app branch. However, something didn't work well. So I just fixed it.

The following commands are what I did

# Clone
git clone https://github.com/eee-c/dart-comics.git
# Go to the repo
cd dart-comics
# Checkout to your_first_dart_app
git checkout your_first_dart_app
# Get libraries
pub get
# Run the app
dart app.dart 

After typing the commands, I got the following page which is just HTML source.

About this PR

What I did are

• Rename name in pubspec.yaml because pub said "name" field must be a valid Dart identifier.
• Add sdks.dart in pubspec.lock (pub command did automatically)
• Add Content-Type (these codes were copied from master branch)
• Generate missing comics.dart.js by dart2js command

Then, you have right page like the following.