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/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) {