dartdoc fails with head analyzer; fields not unique

[jcollins-g]

There is a simple workaround available, but it shouldn't be necessary.

[jcollins-g]

This seems to be a duplicate of #2148; the 'late' field is what shows up, duplicated because the analyzer isn't in non-nullable mode.

dart2-model_special_cases_test.dart: 00:24 +0 -1: Experiments (setUpAll) [E]
dart2-model_special_cases_test.dart:
dart2-model_special_cases_test.dart:   'package:dartdoc/src/model/class.dart': Failed assertion: line 88 pos 16: '!_allElements.containsKey(me.element)': is not true.
dart2-model_special_cases_test.dart:
dart2-model_special_cases_test.dart:   dart:core-patch/errors_patch.dart 42:39                       _AssertionError._doThrowNew
dart2-model_special_cases_test.dart:   dart:core-patch/errors_patch.dart 38:5                        _AssertionError._throwNew
dart2-model_special_cases_test.dart:   package:dartdoc/src/model/class.dart 88:16                    Class.allElements
dart2-model_special_cases_test.dart:   package:dartdoc/src/model/class.dart 112:37                   Class.contains
dart2-model_special_cases_test.dart:   package:dartdoc/src/model/inheritable.dart 51:15              Inheritable.computeCanonicalEnclosingContainer
dart2-model_special_cases_test.dart:   package:dartdoc/src/model/container_member.dart 54:38         ContainerMember.canonicalEnclosingContainer
dart2-model_special_cases_test.dart:   package:dartdoc/src/model/inheritable.dart 26:9               Inheritable.buildCanonicalModelElement
dart2-model_special_cases_test.dart:   package:dartdoc/src/model/model_element.dart 518:34           ModelElement.canonicalModelElement
dart2-model_special_cases_test.dart:   package:dartdoc/src/model/package_graph.dart 121:13           PackageGraph.precacheLocalDocs
dart2-model_special_cases_test.dart:   dart:core-patch/core_patch.dart 146:12                        _SyncIterator.moveNext
dart2-model_special_cases_test.dart:   dart:core-patch/array_patch.dart 47:19                        new List.from
dart2-model_special_cases_test.dart:   dart:core/iterable.dart 400:5                                 Iterable.toList
dart2-model_special_cases_test.dart:   package:dartdoc/src/model/package_graph.dart 69:56            PackageGraph.initializePackageGraph
dart2-model_special_cases_test.dart:   package:dartdoc/src/model/package_builder.dart 51:20          PackageBuilder.buildPackageGraph