Shut up bad analyzer warning

pkg/dev_compiler/web/web_command.dart

 // Copyright (c) 2016, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 @JS()
 library dev_compiler.web.web_command;
 
 import 'dart:async';
 import 'dart:convert';
 import 'dart:html' show HttpRequest;
 
@@ skipping 189 matching lines @@
         moduleCode = module
             .getCode(ModuleFormat.legacy, unit.name, unit.name + '.map',
                 singleOutFile: true)
             .code;
       }
 
       return new CompileResult(
           code: moduleCode, isValid: module.isValid, errors: module.errors);
     };
 
-    return allowInterop(compileFn);
+    // TODO(vsm): Cast is due to https://github.com/dart-lang/sdk/issues/28507
+    return allowInterop(compileFn) as CompileModule;
   }
 }
 
 // Given path, determine corresponding dart library.
 String _moduleForLibrary(source) {
   if (source is InSummarySource) {
     return source.summaryPath.substring(1).replaceAll('.api.ds', '');
   }
   return source.toString().substring(1).replaceAll('.dart', '');
 }
 
 /// Thrown when the input source code has errors.
 class CompileErrorException implements Exception {
   toString() => '\nPlease fix all errors before compiling (warnings are okay).';
 }