Remove use of "--assert-initializer" flag from CONTRIBUTING.md (flutt…

…er#14032)

* Remove mention of --assert-initializer from CONTRIBUTING.md
* Remove options variable as well

CONTRIBUTING.md

@@ -139,7 +139,7 @@ flutter shell. To run the tests, ensure that no devices are connected,
 then navigate to `flutter_tools` and execute:
 
 ```shell
-DART_VM_OPTIONS=--assert-initializer ../../bin/cache/dart-sdk/bin/pub run test
+../../bin/cache/dart-sdk/bin/pub run test
 ```
 
 The pre-built flutter tool runs in release mode with the observatory off by default.
@@ -290,4 +290,4 @@ We build and test Flutter on:
 - Travis ([details](.travis.yml))
 - AppVeyor ([details](appveyor.yml))
 - Chromebots (a.k.a. "recipes", [details](dev/bots/README.md))
-- Devicelab (a.k.a. "cocoon", [details](dev/devicelab/README.md))
+- Devicelab (a.k.a. "cocoon", [details](dev/devicelab/README.md))