assert without --link-to-remote (Field and all Accessors are synthetic)

[jcollins-g]

The bird_flutter package (see #2113) will hit an assert in dartdoc if --link-to-remote is not passed.
The assert does not occur if --link-to-remote is passed. P1 as this case isn't the important one from pub's perspective.

build-bird_flutter-0.0.3:     from bird_flutter.DeltaLoopMode.toString: (file:///var/folders/b8/jg3_spkj02d51bcz3fsb5jjc000b33/T/pubcachewdsQJn/hosted/pub.dartlang.org/bird-0.0.3+5/lib/src/layer2/dart_animation_controller/dart_animation_controller.dart:75:6)
build-bird_flutter-0.0.3:     referred to by bird_flutter: (file:///private/var/folders/b8/jg3_spkj02d51bcz3fsb5jjc000b33/T/pubcachewdsQJn/hosted/pub.dartlang.org/bird_flutter-0.0.3/lib/bird_flutter.dart:1:9)
build-bird_flutter-0.0.3:
build-bird_flutter-0.0.3: dartdoc failed: 'package:dartdoc/src/model/getter_setter_combo.dart': Failed assertion: line 80 pos 14: 'false': Field and accessors can not all be synthetic
build-bird_flutter-0.0.3: dart:core                                                 _AssertionError._throwNew
build-bird_flutter-0.0.3: package:dartdoc/src/model/getter_setter_combo.dart 80:14  GetterSetterCombo.characterLocation
build-bird_flutter-0.0.3: package:dartdoc/src/model/model_element.dart 775:9        ModelElement.location
build-bird_flutter-0.0.3: package:dartdoc/src/model/package_graph.dart 425:67       PackageGraph._warnOnElement
build-bird_flutter-0.0.3: package:dartdoc/src/model/package_graph.dart 289:5        PackageGraph.warnOnElement
build-bird_flutter-0.0.3: package:dartdoc/src/model/model_element.dart 1057:18      ModelElement.warn
build-bird_flutter-0.0.3: package:dartdoc/src/model/model_element.dart 683:13       ModelElement.canonicalLibrary
build-bird_flutter-0.0.3: package:dartdoc/src/model/model_element.dart 742:20       ModelElement.isCanonical
build-bird_flutter-0.0.3: package:dartdoc/src/model/enum.dart 94:20                 EnumField.isCanonical
build-bird_flutter-0.0.3: package:dartdoc/src/model/accessor.dart 96:42             Accessor.isCanonical
build-bird_flutter-0.0.3: package:dartdoc/src/model/class.dart 165:41               Class.allCanonicalModelElements.<fn>
build-bird_flutter-0.0.3: dart:core                                                 Iterable.toList
build-bird_flutter-0.0.3: package:dartdoc/src/model/class.dart 165:54               Class.allCanonicalModelElements
build-bird_flutter-0.0.3: package:dartdoc/src/model/inheritable.dart 31:13          Inheritable.buildCanonicalModelElement
build-bird_flutter-0.0.3: package:dartdoc/src/model/model_element.dart 518:34       ModelElement.canonicalModelElement
build-bird_flutter-0.0.3: package:dartdoc/src/model/package_graph.dart 121:13       PackageGraph.precacheLocalDocs
build-bird_flutter-0.0.3: dart:core                                                 Iterable.toList
build-bird_flutter-0.0.3: package:dartdoc/src/model/package_graph.dart 69:56        PackageGraph.initializePackageGraph
build-bird_flutter-0.0.3: dart:async                                                _AsyncAwaitCompleter.start
build-bird_flutter-0.0.3: package:dartdoc/src/model/package_graph.dart 61:38        PackageGraph.initializePackageGraph
build-bird_flutter-0.0.3: package:dartdoc/src/model/package_builder.dart 51:20      PackageBuilder.buildPackageGraph
build-bird_flutter-0.0.3: dart:async                                                _completeOnAsyncReturn
build-bird_flutter-0.0.3: package:dartdoc/src/model/package_builder.dart            PackageBuilder.getLibraries
build-bird_flutter-0.0.3: dart:async                                                _completeOnAsyncReturn
build-bird_flutter-0.0.3: package:dartdoc/src/model/package_builder.dart            PackageBuilder._parseLibraries
build-bird_flutter-0.0.3:

@isoos

[jcollins-g]

Fixed by other PRs; no longer reproducible at head.