stop polling when switching networks

app/scripts/controllers/network.js

@@ -53,6 +53,7 @@ module.exports = class NetworkController extends EventEmitter {
     this.providerInit = newInit
 
     this._provider.removeAllListeners()
+    this._provider.stop()
     this.provider = MetaMaskProvider(newInit)
     // apply the listners created by other controllers
     Object.keys(this._providerListeners).forEach((key) => {

test/unit/network-contoller-test.js

@@ -12,7 +12,11 @@ describe('# Network Controller', function () {
       },
     })
     // stub out provider
-    networkController._provider = new EventEmitter()
+    networkController._provider = new Proxy(new EventEmitter(), {
+      get: (obj, name) => {
+        return () => {}
+      },
+    })
     networkController.providerInit = {
       getAccounts: () => {},
     }