regression: widespread no canonical found warnings in dartdoc for flutter

[jcollins-g]

Sometime before 0.29.2 (probably pre 0.29.0 but it is difficult to tell since it's hard to run very old versions of dartdoc now for apples to apples comparisons), flutter docs starting having trouble linking to canonicalModelElements. Many warnings of this type are displayed but there definitely are canonical references so we shouldn't be giving up here.

flutter-docs-original:   warning: no canonical library found for flutter_localizations.GlobalMaterialLocalizations.alertDialogLabel, not linking, from flutter_localizations.GlobalMaterialLocalizations.alertDialogLabe
l: (file:///private/var/folders/b8/jg3_spkj02d51bcz3fsb5jjc000b33/T/dartdoc-comparison-flutter5LTqFQ/packages/flutter/lib/src/material/material_localizations.dart:166:14)
flutter-docs-original:   warning: no canonical library found for flutter_localizations.GlobalMaterialLocalizations.anteMeridiemAbbreviation, not linking, from flutter_localizations.GlobalMaterialLocalizations.anteMer
idiemAbbreviation: (file:///private/var/folders/b8/jg3_spkj02d51bcz3fsb5jjc000b33/T/dartdoc-comparison-flutter5LTqFQ/packages/flutter/lib/src/material/material_localizations.dart:132:14)
flutter-docs-original:   warning: no canonical library found for flutter_localizations.GlobalMaterialLocalizations.backButtonTooltip, not linking, from flutter_localizations.GlobalMaterialLocalizations.backButtonTool
tip: (file:///private/var/folders/b8/jg3_spkj02d51bcz3fsb5jjc000b33/T/dartdoc-comparison-flutter5LTqFQ/packages/flutter/lib/src/material/material_localizations.dart:58:14)
flutter-docs-original:   warning: no canonical library found for flutter_localizations.GlobalMaterialLocalizations.cancelButtonLabel, not linking, from flutter_localizations.GlobalMaterialLocalizations.cancelButtonLa
bel: (file:///private/var/folders/b8/jg3_spkj02d51bcz3fsb5jjc000b33/T/dartdoc-comparison-flutter5LTqFQ/packages/flutter/lib/src/material/material_localizations.dart:105:14)
flutter-docs-original:   warning: no canonical library found for flutter_localizations.GlobalMaterialLocalizations.closeButtonLabel, not linking, from flutter_localizations.GlobalMaterialLocalizations.closeButtonLabe
l: (file:///private/var/folders/b8/jg3_spkj02d51bcz3fsb5jjc000b33/T/dartdoc-comparison-flutter5LTqFQ/packages/flutter/lib/src/material/material_localizations.dart:108:14)
flutter-docs-original:   warning: no canonical library found for flutter_localizations.GlobalMaterialLocalizations.closeButtonTooltip, not linking, from flutter_localizations.GlobalMaterialLocalizations.closeButtonTo
oltip: (file:///private/var/folders/b8/jg3_spkj02d51bcz3fsb5jjc000b33/T/dartdoc-comparison-flutter5LTqFQ/packages/flutter/lib/src/material/material_localizations.dart:61:14)
flutter-docs-original:   warning: no canonical library found for flutter_localizations.GlobalMaterialLocalizations.collapsedIconTapHint, not linking, from flutter_localizations.GlobalMaterialLocalizations.collapsedIc
onTapHint: (file:///private/var/folders/b8/jg3_spkj02d51bcz3fsb5jjc000b33/T/dartdoc-comparison-flutter5LTqFQ/packages/flutter/lib/src/material/material_localizations.dart:310:14)

[jcollins-g]

The primary symptom of this will be random dead links where we expect links to work.