Update to take into account Dart SDK 1.20

Dart SDK 1.20 no longer creates `packages` directories. Added comments to `pub` files section to differentiate  what is needed for >= SDK 1.20 and what is additionally needed for older SDK versions.

Dart.gitignore

 # See https://www.dartlang.org/tools/private-files.html
 
 # Files and directories created by pub
-.buildlog
+
+# SDK 1.20 and later (no longer creates packages directories)
 .packages
-.project
 .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