Add supported Luci builders for packages repo (#184)

* add luci try builders

* add readme

* add code to validate json contents

* remove json check dart

* use generic name

dev/README.md

@@ -0,0 +1,19 @@
+This directory contains resources that the Flutter team uses during 
+the development of packages.
+
+## Luci builder file
+`try_builders.json` contains the supported luci try builders 
+for packages. It follows format:
+```json
+{
+    "builders":[
+        {
+            "name":"xxx",
+            "repo":"packages"
+        }
+    ]
+}
+```
+This file will be mainly used in [`flutter/cocoon`](https://github.com/flutter/cocoon)
+to update luci task statuses of packages pre-submit tests.
+

dev/try_builders.json

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+{
+   "builders":[
+      {
+         "name":"fuchsia_ctl",
+         "repo":"packages"
+      }
+   ]
+}