Create a FakeTetherService, which stubs out TetherService for development.

chrome/browser/chromeos/tether/tether_service.h

Index: chrome/browser/chromeos/tether/tether_service.h
diff --git a/chrome/browser/chromeos/tether/tether_service.h b/chrome/browser/chromeos/tether/tether_service.h
index 814f4bf1bdb88237a153f9aec1734ac4b95537a5..a87176291979b759501fe5d5e1186075e266f043 100644
--- a/chrome/browser/chromeos/tether/tether_service.h
+++ b/chrome/browser/chromeos/tether/tether_service.h
@@ -47,10 +47,10 @@ class TetherService : public KeyedService,
   // Attempt to start the Tether module. Only succeeds if all conditions to
   // reach chromeos::NetworkStateHandler::TechnologyState::ENABLED are reached.
   // Should only be called once a user is logged in.
-  void StartTetherIfEnabled();
+  virtual void StartTetherIfEnabled();
 
   // Stop the Tether module.
-  void StopTether();
+  virtual void StopTether();
 
  protected:
   // KeyedService:
@@ -79,21 +79,24 @@ class TetherService : public KeyedService,
   // Callback when the controlling pref changes.
   void OnPrefsChanged();
 
+  // Whether Tether hosts are available.
+  virtual bool HasSyncedTetherHosts() const;
+
   virtual void UpdateTetherTechnologyState();
+  chromeos::NetworkStateHandler::TechnologyState GetTetherTechnologyState();
+
+  chromeos::NetworkStateHandler* network_state_handler() {
+    return network_state_handler_;
+  }
 
  private:
   friend class TetherServiceTest;
 
-  chromeos::NetworkStateHandler::TechnologyState GetTetherTechnologyState();
-
   void OnBluetoothAdapterFetched(
       scoped_refptr<device::BluetoothAdapter> adapter);
 
   bool IsBluetoothAvailable() const;
 
-  // Whether Tether hosts are available.
-  bool HasSyncedTetherHosts() const;
-
   // Whether the Tether feature has been enabled via an about or command line
   // flag.
   bool IsFeatureFlagEnabled() const;