Remove instanceof checks

Summary: Replaced `instanceof` checks with `isSandyPlugin` utility. That is cleaner to read and makes it easier to find places where we make exceptions for Sandy plugins

Reviewed By: jknoxville

Differential Revision: D22206707

fbshipit-source-id: b44a1b585424f3b9bf0d7ce200c34107f03ed55e

desktop/app/src/plugin.tsx

@@ -37,6 +37,12 @@ export type ClientPluginDefinition =
 export type ClientPluginMap = Map<string, ClientPluginDefinition>;
 export type DevicePluginMap = Map<string, DevicePluginDefinition>;
 
+export function isSandyPlugin(
+  plugin?: PluginDefinition,
+): plugin is SandyPluginDefinition {
+  return plugin instanceof SandyPluginDefinition;
+}
+
 // This function is intended to be called from outside of the plugin.
 // If you want to `call` from the plugin use, this.client.call
 export function callClient(