Add keywords field to pubspec.yaml

[anders-sandholm]

From @mnordine on August 11, 2017 12:59

Search results on https://pub.dartlang.org/ could be improved with the addition of keywords for packages. It could also improve the home page by listing packages via popular keywords. For prior art, see https://crates.io/ and http://doc.crates.io/manifest.html

From the previous link:

# This is a list of up to five keywords that describe this crate. Keywords
# are searchable on crates.io, and you may choose any words that would
# help someone find this crate.v
keywords = ["...", "..."]

Copied from original issue: dart-lang/sdk#30418

[nex3]

This issue was moved to dart-lang/pub-dev#368