add web_dashboard API (#333)

* add web_dashboard sample

* add docs

* address code review comments

* restructure web_dashboard

* set up provider and mock service

* add copyright headers, use relative imports

* add API class, add tests

* fmt

* rename services -> API, remove data library

* use new API in app

* add stream to items api

* convert from StreamBuilder to StreamProvider

* add subscription to Entry API

* rename API classes

* address comments

* Update README.md

* update README

* remove routing_demo

experimental/web_dashboard/.gitignore

@@ -0,0 +1,37 @@
+# Miscellaneous
+*.class
+*.log
+*.pyc
+*.swp
+.DS_Store
+.atom/
+.buildlog/
+.history
+.svn/
+
+# IntelliJ related
+*.iml
+*.ipr
+*.iws
+.idea/
+
+# The .vscode folder contains launch configuration and tasks you configure in
+# VS Code which you may wish to be included in version control, so this line
+# is commented out by default.
+#.vscode/
+
+# Flutter/Dart/Pub related
+**/doc/api/
+.dart_tool/
+.flutter-plugins
+.flutter-plugins-dependencies
+.packages
+.pub-cache/
+.pub/
+/build/
+
+# Web related
+lib/generated_plugin_registrant.dart
+
+# Exceptions to above rules.
+!/packages/flutter_tools/test/data/dart_dependencies_test/**/.packages