Pub command to create or edit a pubspec

[DartBot]

This issue was originally filed by @seaneagan

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Just like npm init. Could be pub init, or maybe something which portrays that you can use it on an existing pubspec like pub spec. Might be nice to be able to pass arguments like pub spec --name foo --description "Foo bar", especially if pub gets tab completion in the future.

[floitschG]

Removed Type-Defect label.
Added Type-Enhancement, Area-Pub, Triaged labels.

[nex3]

Added Duplicate label.
Marked as being merged into #2432.

[DartBot]

This issue has been moved to dart-lang/pub#1135.