Create a FakeTetherService, which stubs out TetherService for development.

chrome/browser/chromeos/tether/tether_service.cc

Index: chrome/browser/chromeos/tether/tether_service.cc
diff --git a/chrome/browser/chromeos/tether/tether_service.cc b/chrome/browser/chromeos/tether/tether_service.cc
index e0dd94fb4b1fd24d64c816e0c5b27eda2f492663..797e978e9493975084f819dd189998e74be0db0e 100644
--- a/chrome/browser/chromeos/tether/tether_service.cc
+++ b/chrome/browser/chromeos/tether/tether_service.cc
@@ -26,7 +26,10 @@
 
 // static
 TetherService* TetherService::Get(Profile* profile) {
-  return TetherServiceFactory::GetForBrowserContext(profile);
+  if (IsFeatureFlagEnabled())
+    return TetherServiceFactory::GetForBrowserContext(profile);
+
+  return nullptr;
 }
 
 // static
@@ -36,6 +39,12 @@ void TetherService::RegisterProfilePrefs(PrefRegistrySimple* registry) {
   chromeos::tether::Initializer::RegisterProfilePrefs(registry);
 }
 
+// static
+bool TetherService::IsFeatureFlagEnabled() {
+  return base::CommandLine::ForCurrentProcess()->HasSwitch(
+      chromeos::switches::kEnableTether);
+}
+
 TetherService::TetherService(
     Profile* profile,
     chromeos::PowerManagerClient* power_manager_client,
@@ -176,6 +185,12 @@ void TetherService::OnPrefsChanged() {
   UpdateTetherTechnologyState();
 }
 
+bool TetherService::HasSyncedTetherHosts() const {
+  return !cryptauth_service_->GetCryptAuthDeviceManager()
+              ->GetTetherHosts()
+              .empty();
+}
+
 void TetherService::UpdateTetherTechnologyState() {
   chromeos::NetworkStateHandler::TechnologyState tether_technology_state =
       GetTetherTechnologyState();
@@ -193,7 +208,7 @@ void TetherService::UpdateTetherTechnologyState() {
 chromeos::NetworkStateHandler::TechnologyState
 TetherService::GetTetherTechnologyState() {
   if (shut_down_ || suspended_ || session_manager_client_->IsScreenLocked() ||
-      !IsFeatureFlagEnabled() || !HasSyncedTetherHosts()) {
+      !HasSyncedTetherHosts()) {
     return chromeos::NetworkStateHandler::TechnologyState::
         TECHNOLOGY_UNAVAILABLE;
   } else if (!IsAllowedByPolicy()) {
@@ -220,17 +235,6 @@ void TetherService::OnBluetoothAdapterFetched(
   UpdateTetherTechnologyState();
 }
 
-bool TetherService::HasSyncedTetherHosts() const {
-  return !cryptauth_service_->GetCryptAuthDeviceManager()
-              ->GetTetherHosts()
-              .empty();
-}
-
-bool TetherService::IsFeatureFlagEnabled() const {
-  return base::CommandLine::ForCurrentProcess()->HasSwitch(
-      chromeos::switches::kEnableTether);
-}
-
 bool TetherService::IsBluetoothAvailable() const {
   return adapter_.get() && adapter_->IsPresent() && adapter_->IsPowered();
 }