WIP: Dart release 2.0 (#6927)

* chore: make flatc artifacts from CI executable

* chore: prepare dart 2.0.0 release

* refactor: update description in pubspec.yaml to make pub.dev happy

"The package description is too long.
Search engines display only the first part of the description. Try to keep the value of the description field in your package's pubspec.yaml file between 60 and 180 characters."

.github/workflows/build.yml

@@ -23,6 +23,10 @@ jobs:
       run: make -j4
     - name: test
       run: ./flattests
+    - name: make flatc executable
+      run: |
+        chmod +x flatc
+        ./flatc --version
     - name: upload build artifacts
       uses: actions/upload-artifact@v1
       with:
@@ -79,6 +83,10 @@ jobs:
         echo $info | grep "universal binary with 2 architectures"
     - name: test
       run: _build/Release/flattests
+    - name: make flatc executable
+      run: |
+        chmod +x _build/Release/flatc
+        ./_build/Release/flatc --version
     - name: upload build artifacts
       uses: actions/upload-artifact@v1
       with: