LateInitializationError on codegen

**Steps to Reproduce**

Happens randomly after running `flutter packages pub run build_runner build --delete-conflicting-outputs`

**Result**
```
[SEVERE] hive_generator:hive_generator on lib/social_media_menu_dialog.dart:

LateInitializationError: Field 'exports' has not been initialized.
#0 LibraryReader.exports (package:analyzer/src/summary2/bundle_reader.dart)
#1 LinkedElementFactory.exportsOfLibrary (package:analyzer/src/summary2/linked_element_factory.dart:156:20)
#2 LinkedElementFactory.buildExportNamespace (package:analyzer/src/summary2/linked_element_factory.dart:51:30)
#3 LibraryElementImpl.exportNamespace (package:analyzer/src/dart/element/element.dart:3628:42)
#4 NamespaceBuilder._getExportMapping (package:analyzer/src/dart/resolver/scope.dart:271:20)
#5 NamespaceBuilder.getImportedElements (package:analyzer/src/dart/resolver/scope.dart:211:15)
#6 new PrefixScope (package:analyzer/src/dart/element/scope.dart:181:48)
#7 new _LibraryImportScope (package:analyzer/src/dart/element/scope.dart:295:28)
#8 new LibraryScope (package:analyzer/src/dart/element/scope.dart:106:15)
#9 LibraryElementImpl.scope (package:analyzer/src/dart/element/element.dart:3840:23)
#10 LibraryBuilder.buildScope (package:analyzer/src/summary2/library_builder.dart:134:21)
#11 Linker._computeLibraryScopes (package:analyzer/src/summary2/link.dart:170:15)
#12 Linker._buildOutlines (package:analyzer/src/summary2/link.dart:98:5)
#13 Linker.link (package:analyzer/src/summary2/link.dart:78:5)
#14 link (package:analyzer/src/summary2/link.dart:35:10)
#15 LibraryContext.load2.loadBundle (package:analyzer/src/dart/analysis/library_context.dart:172:24)
#16 LibraryContext.load2. (package:analyzer/src/dart/analysis/library_context.dart:203:17)
#17 PerformanceLog.run (package:analyzer/src/dart/analysis/performance_logger.dart:32:15)
#18 LibraryContext.load2 (package:analyzer/src/dart/analysis/library_context.dart:201:12)
#19 AnalysisDriver._createLibraryContext (package:analyzer/src/dart/analysis/driver.dart:1848:22)
#20 AnalysisDriver._computeUnitElement. (package:analyzer/src/dart/analysis/driver.dart:1760:28)
#21 PerformanceLog.run (package:analyzer/src/dart/analysis/performance_logger.dart:32:15)
#22 AnalysisDriver._computeUnitElement (package:analyzer/src/dart/analysis/driver.dart:1758:20)
#23 AnalysisDriver.performWork (package:analyzer/src/dart/analysis/driver.dart:1342:35)
#24 AnalysisDriverScheduler._run (package:analyzer/src/dart/analysis/driver.dart:2334:24)

```

`social_media_menu_dialog.dart` doesn´t import `package:hive/hive.dart` and doesn´t even use Hive (nor Mobx or JSON which also require codegen).

I also use:
```
mobx_codegen: ^2.0.1+3
build_runner: ^2.0.4
json_serializable: ^4.1.1
effective_dart: ^1.3.1
flutter_gen: ^3.1.1
```

**Version**
- Platform: Windows, Android
- Flutter version: 2.2.3
- Hive version: 3.1.1