Drastically simplify .gitignore for Dart

Our typical workflow has JS and related build outputs in the `build` directory.

Most users are on new SDKs, so we don't have to worry about these ignores.
Most users don't generate docs locally. If they do, it's easy to add the entry to their .gitignore later.

Dart.gitignore

 # See https://www.dartlang.org/tools/private-files.html
 
 # Files and directories created by pub
-
-# SDK 1.20 and later (no longer creates packages directories)
 .packages
 .pub/
 build/
-
-# Older SDK versions
-# (Include if the minimum SDK version specified in pubsepc.yaml is earlier than 1.20)
-.project
-.buildlog
-**/packages/
-
-
-# Files created by dart2js
-# (Most Dart developers will use pub build to compile Dart, use/modify these 
-#  rules if you intend to use dart2js directly
-#  Convention is to use extension '.dart.js' for Dart compiled to Javascript to
-#  differentiate from explicit Javascript files)
-*.dart.js
-*.part.js
-*.js.deps
-*.js.map
-*.info.json
-
-# Directory created by dartdoc
-doc/api/
-
-# Don't commit pubspec lock file 
-# (Library packages only! Remove pattern if developing an application package)
+# If you're building an application, you may want to check-in your pubspec.lock.
 pubspec.lock