anchor.diff:  
      testWithoutContext('Cache does not attempt to delete already missing stamp files', () {
       final FileSystem fileSystem = MemoryFileSystem.test();
  -    final ArtifactSet artifactSet = MockIosUsbArtifacts();
  +    final FakeIosUsbArtifacts artifactSet = FakeIosUsbArtifacts();
       final BufferLogger logger = BufferLogger.test();
   
  -    when(artifactSet.stampName).thenReturn('STAMP');
  +    artifactSet.stampName = 'STAMP';
       final Cache cache = Cache(
         artifacts: <ArtifactSet>[
           artifactSet,
  
positive.code:       );
  
        await cache.updateAll(<DevelopmentArtifact>{
          null,
        });
        verifyNever(artifact1.update(any, any, any, any));
        verify(artifact2.update(any, any, any, any));
      });
  
      testWithoutContext("getter dyLdLibEntry concatenates the output of each artifact's dyLdLibEntry getter", () async {
        final IosUsbArtifacts artifact1 = MockIosUsbArtifacts();
        final IosUsbArtifacts artifact2 = MockIosUsbArtifacts();
        final IosUsbArtifacts artifact3 = MockIosUsbArtifacts();
        when(artifact1.environment)
            .thenReturn(<String, String>{
              'DYLD_LIBRARY_PATH': '/path/to/alpha:/path/to/beta',
            });
        when(artifact2.environment)
            .thenReturn(<String, String>{
              'DYLD_LIBRARY_PATH': '/path/to/gamma:/path/to/delta:/path/to/epsilon',
            });
        when(artifact3.environment)
            .thenReturn(<String, String>{
              'DYLD_LIBRARY_PATH': '',
            });
        final Cache cache = Cache.test(
          artifacts: <CachedArtifact>[artifact1, artifact2, artifact3],
          processManager: FakeProcessManager.any(),
  
negative.code:         logger: logger,
        );
        await expectLater(
          () => cache.updateAll(<DevelopmentArtifact>{DevelopmentArtifact.universal}),
          throwsException,
        );
        expect(artifact1.didUpdate, true);
        // Don't continue when retrieval fails.
        expect(artifact2.didUpdate, false);
        expect(
          logger.errorText,
          contains('https://flutter.dev/community/china'),
  