Check for "empty structs" in Dart code

[dkastl]

The following warning is shown when compiling with a recent version of Flutter.

../../../snap/flutter/common/flutter/.pub-cache/hosted/pub.dartlang.org/win32-1.7.3/lib/src/structs.dart:1083:7: Info: Struct 'ENUMLOGFONTEX' is empty. Support for empty structs is deprecated and will be removed in the next stable version of Dart. Use Opaque instead.
class ENUMLOGFONTEX extends Struct {                                    
      ^                                                                 
../../../snap/flutter/common/flutter/.pub-cache/hosted/pub.dartlang.org/win32-1.7.3/lib/src/structs.dart:2807:7: Info: Struct 'BLUETOOTH_PIN_INFO' is empty. Support for empty structs is deprecated and will be removed in the next stable version of Dart. Use Opaque instead.
class BLUETOOTH_PIN_INFO extends Struct {                               
      ^                                                                 
../../../snap/flutter/common/flutter/.pub-cache/hosted/pub.dartlang.org/win32-1.7.3/lib/src/structs.dart:2932:7: Info: Struct 'EXCEPINFO' is empty. Support for empty structs is deprecated and will be removed in the next stable version of Dart. Use Opaque instead.
class EXCEPINFO extends Struct {}                                       
      ^                                                                 
../../../snap/flutter/common/flutter/.pub-cache/hosted/pub.dartlang.org/win32-1.7.3/lib/src/structs.dart:2938:7: Info: Struct 'PROPERTYKEY' is empty. Support for empty structs is deprecated and will be removed in the next stable version of Dart. Use Opaque instead.
class PROPERTYKEY extends Struct {}                                     
      ^                                                                 
../../../snap/flutter/common/flutter/.pub-cache/hosted/pub.dartlang.org/win32-1.7.3/lib/src/structs.dart:2945:7: Info: Struct 'PROPVARIANT' is empty. Support for empty structs is deprecated and will be removed in the next stable version of Dart. Use Opaque instead.
class PROPVARIANT extends Struct {}                                     
      ^                                                                 
../../../snap/flutter/common/flutter/.pub-cache/hosted/pub.dartlang.org/win32-1.7.3/lib/src/structs.dart:2950:7: Info: Struct 'SAFEARRAY' is empty. Support for empty structs is deprecated and will be removed in the next stable version of Dart. Use Opaque instead.
class SAFEARRAY extends Struct {}                                       
      ^                                                                 
../../../snap/flutter/common/flutter/.pub-cache/hosted/pub.dartlang.org/win32-1.7.3/lib/src/structs.dart:2957:7: Info: Struct 'CLSID' is empty. Support for empty structs is deprecated and will be removed in the next stable version of Dart. Use Opaque instead.
class CLSID extends Struct {}                                           
      ^                                                                 
../../../snap/flutter/common/flutter/.pub-cache/hosted/pub.dartlang.org/win32-1.7.3/lib/src/structs.dart:2964:7: Info: Struct 'STATSTG' is empty. Support for empty structs is deprecated and will be removed in the next stable version of Dart. Use Opaque instead.
class STATSTG extends Struct {}                                         
      ^                                                                 
../../../snap/flutter/common/flutter/.pub-cache/hosted/pub.dartlang.org/win32-1.7.3/lib/src/structs.dart:2971:7: Info: Struct 'NLM_SIMULATED_PROFILE_INFO' is empty. Support for empty structs is deprecated and will be removed in the next stable version of Dart. Use Opaque instead.
class NLM_SIMULATED_PROFILE_INFO extends Struct {}                      
      ^                                                                 
../../../snap/flutter/common/flutter/.pub-cache/hosted/pub.dartlang.org/ffi-0.1.3/lib/src/utf8.dart:23:7: Info: Struct 'Utf8' is empty. Support for empty structs is deprecated and will be removed in the next stable version of Dart. Use Opaque instead.
class Utf8 extends Struct {                                             
      ^                                                                 
../../../snap/flutter/common/flutter/.pub-cache/hosted/pub.dartlang.org/ffi-0.1.3/lib/src/utf16.dart:16:7: Info: Struct 'Utf16' is empty. Support for empty structs is deprecated and will be removed in the next stable version of Dart. Use Opaque instead.
class Utf16 extends Struct {                                            
      ^                                                

This may break compilation in upcoming versions.