Add missing returns.

As of `1.18.0-dev-0`, these cases will get flagged.  In the meantime, the
diff --git a/packages/flutter_tools/lib/src/application_package.dart b/packages/flutter_tools/lib/src/application_package.dart
index dcd55eb..2810901 100644
--- a/packages/flutter_tools/lib/src/application_package.dart
+++ b/packages/flutter_tools/lib/src/application_package.dart
@@ -134,6 +134,7 @@
     case TargetPlatform.linux_x64:
       return null;
   }
+  return null;
 }
 
 class ApplicationPackageStore {
@@ -156,5 +157,6 @@
       case TargetPlatform.linux_x64:
         return null;
     }
+    return null;
   }
 }
diff --git a/packages/flutter_tools/lib/src/build_info.dart b/packages/flutter_tools/lib/src/build_info.dart
index c8d600e..c599cba 100644
--- a/packages/flutter_tools/lib/src/build_info.dart
+++ b/packages/flutter_tools/lib/src/build_info.dart
@@ -53,6 +53,7 @@
       return 'linux-x64';
   }
   assert(false);
+  return null;
 }
 
 enum TargetPlatform {
@@ -80,6 +81,7 @@
       return 'linux-x64';
   }
   assert(false);
+  return null;
 }
 
 TargetPlatform getTargetPlatformForName(String platform) {
diff --git a/packages/flutter_tools/lib/src/ios/devices.dart b/packages/flutter_tools/lib/src/ios/devices.dart
index 603ebc1..75804dd 100644
--- a/packages/flutter_tools/lib/src/ios/devices.dart
+++ b/packages/flutter_tools/lib/src/ios/devices.dart
@@ -235,7 +235,6 @@
     } else {
       return false;
     }
-    return false;
   }
 
   @override