anchor.diff:          '/foo/a.dart',
           '/foo/b.dart',
         ]));
  -    }, overrides: <Type, Generator>{ FileSystem: () => fs});
  +    }, overrides: <Type, Generator>{ FileSystem: () => fs });
   
       testUsingContext('trims extra spaces between files', () async {
         await fs.file('a.d').writeAsString('snapshot.d: /foo/a.dart    /foo/b.dart  /foo/c.dart');
  
positive.code:           () => new Checksum.fromFiles(null, TargetPlatform.ios, <String>['a.dart', 'b.dart'].toSet()),
            throwsA(anything),
          );
        }, overrides: <Type, Generator>{ FileSystem: () => fs});
  
        testUsingContext('does not throw if any target platform is null', () async {
          await fs.file('a.dart').create();
  
negative.code:           new Checksum.fromFiles(BuildMode.debug, null, <String>['a.dart'].toSet()),
            isNotNull,
          );
        }, overrides: <Type, Generator>{ FileSystem: () => fs });
  
        testUsingContext('populates checksums for valid files', () async {
          await fs.file('a.dart').writeAsString('This is a');
  