dart2: update to work with dart 2.3.0

[ircecho]

PR checklist

• Read the contribution guidelines.
• Ran the shell script under ./bin/ to update Petstore sample so that CIs can verify the change. (For instance, only need to run ./bin/{LANG}-petstore.sh, ./bin/openapi3/{LANG}-petstore.sh if updating the {LANG} (e.g. php, ruby, python, etc) code generator or {LANG} client's mustache templates). Windows batch files can be found in .\bin\windows\. If contributing template-only or documentation-only changes which will change sample output, be sure to build the project first.
• Filed the PR against the correct branch: master, 4.1.x, 5.0.x. Default: master.
• Copied the technical committee to review the pull request if your PR is targeting a particular programming language.

@ircecho (2017/07) @swipesight (2018/09) @jaumard (2018/09)

Description of the PR

(details of the change, additional tests that have been done, reference to the issue for tracking, etc)

Dart 2.3.0 no longer works with http 0.11.+

[wing328]

The change looks reasonable to me.

[wing328]

SNAPSHOT version with this enhancement: https://oss.sonatype.org/content/repositories/snapshots/org/openapitools/openapi-generator-cli/4.0.1-SNAPSHOT/openapi-generator-cli-4.0.1-20190515.161655-6.jar

Tweet: https://twitter.com/oas_generator/status/1128955859545968640

[wing328]

@ircecho thanks again for your contribution, which has been included in the v4.0.1 release (https://twitter.com/oas_generator/status/1135534738658062336)