Clean up after r39484 and r39486.

pkg/args/CHANGELOG.md

+## 0.12.0+2
+
+* Widen the version constraint on the `collection` package.
+
 ## 0.12.0+1
 
 * Remove the documentation link from the pubspec so this is linked to
   pub.dartlang.org by default.
 
 ## 0.12.0
 
 * Removed public constructors for `ArgResults` and `Option`.
  
 * `ArgResults.wasParsed()` can be used to determine if an option was actually
   parsed or the default value is being returned.
 
 * Replaced `isFlag` and `allowMultiple` fields in the `Option` class with a
   three-value `OptionType` enum.
   
 * Options may define `valueHelp` which will then be shown in the usage.
 
 ## 0.11.0
 
 * Move handling trailing options from `ArgParser.parse()` into `ArgParser`
   itself. This lets subcommands have different behavior for how they handle
   trailing options.
 
 ## 0.10.0+2
 
 * Usage ignores hidden options when determining column widths.