[Dart 2] Add support for Dart 2 (#754)

* Add an option for Dart2

* add dart2 samples, update travis

* fix dart installation

* Upper constraints on the SDK version

* Update dependencies

* supportDart2 option can now be passed through --additional-properties

* Update petstore tests

* Update dart2-petstore.sh

* Running tests on Dart VM

* Fixed JSON deserialization bugs

* Fixed missing initialization of postBody

* Run bin/dart2-petstore.sh to regenerate libraries

* Update pom.xml

* Added SDK version constraints in pubspec.mustache

* Run bin/dart2-petstore.sh to regenerate libraries

* move dart2 test to the end

samples/client/petstore/dart2/openapi/git_push.sh

@@ -0,0 +1,52 @@
+#!/bin/sh
+# ref: https://help.github.com/articles/adding-an-existing-project-to-github-using-the-command-line/
+#
+# Usage example: /bin/sh ./git_push.sh wing328 openapi-pestore-perl "minor update"
+
+git_user_id=$1
+git_repo_id=$2
+release_note=$3
+
+if [ "$git_user_id" = "" ]; then
+    git_user_id="GIT_USER_ID"
+    echo "[INFO] No command line input provided. Set \$git_user_id to $git_user_id"
+fi
+
+if [ "$git_repo_id" = "" ]; then
+    git_repo_id="GIT_REPO_ID"
+    echo "[INFO] No command line input provided. Set \$git_repo_id to $git_repo_id"
+fi
+
+if [ "$release_note" = "" ]; then
+    release_note="Minor update"
+    echo "[INFO] No command line input provided. Set \$release_note to $release_note"
+fi
+
+# Initialize the local directory as a Git repository
+git init
+
+# Adds the files in the local repository and stages them for commit.
+git add .
+
+# Commits the tracked changes and prepares them to be pushed to a remote repository. 
+git commit -m "$release_note"
+
+# Sets the new remote
+git_remote=`git remote`
+if [ "$git_remote" = "" ]; then # git remote not defined
+
+    if [ "$GIT_TOKEN" = "" ]; then
+        echo "[INFO] \$GIT_TOKEN (environment variable) is not set. Using the git credential in your environment."
+        git remote add origin https://github.com/${git_user_id}/${git_repo_id}.git
+    else
+        git remote add origin https://${git_user_id}:${GIT_TOKEN}@github.com/${git_user_id}/${git_repo_id}.git
+    fi
+
+fi
+
+git pull origin master
+
+# Pushes (Forces) the changes in the local repository up to the remote repository
+echo "Git pushing to https://github.com/${git_user_id}/${git_repo_id}.git"
+git push origin master 2>&1 | grep -v 'To https'
+