Update Dart.gitignore

Modify patterns to explicitly match directories

Modified patterns to match files created by tool in Dart SDK 1.12:
 - Added patterns matching files listed in https://www.dartlang.org/tools/private-files.html
 - Added *.js, removed *.js_, *.dart.js to match files created by dart2js

Dart.gitignore

-# Don’t commit the following directories created by pub.
+# Files and directories created by pub
 .buildlog
+.packages
+.project
 .pub/
 build/
-packages
-.packages
+**/packages/
 
-# Or the files created by dart2js.
-*.dart.js
-*.js_
+# Files created by dart2js
+*.js
 *.js.deps
 *.js.map
 
-# Or the files created by dartdoc.
+# Directory created by dartdoc
 doc/
 
-# Don't commit pubspec lock file. (Library packages only! Remove pattern if developing an application package.)
+# Don't commit pubspec lock file (Library packages only! Remove pattern if developing an application package)
 pubspec.lock