fix test for null flutter root (#44468)

packages/flutter_tools/lib/src/commands/assemble.dart

@@ -87,11 +87,16 @@ class AssembleCommand extends FlutterCommand {
     if (futterProject == null) {
       return const <CustomDimensions, String>{};
     }
+    try {
       final Environment localEnvironment = environment;
       return <CustomDimensions, String>{
         CustomDimensions.commandBuildBundleTargetPlatform: localEnvironment.defines['TargetPlatform'],
         CustomDimensions.commandBuildBundleIsModule: '${futterProject.isModule}',
       };
+    } catch (err) {
+      // We've failed to send usage.
+    }
+    return const <CustomDimensions, String>{};
   }
 
   /// The target we are building.

packages/flutter_tools/test/commands.shard/hermetic/assemble_test.dart

@@ -18,15 +18,13 @@ import '../../src/testbed.dart';
 void main() {
   Testbed testbed;
   MockBuildSystem mockBuildSystem;
-
-  setUpAll(() {
   Cache.disableLocking();
-  });
 
   setUp(() {
     mockBuildSystem = MockBuildSystem();
     testbed = Testbed(overrides: <Type, Generator>{
       BuildSystem: ()  => mockBuildSystem,
+      Cache: () => FakeCache(),
     });
   });