Remove Library.sentinel. (#4155)

Use `null` instead of a sentinel value that throws on all member accessses.

That lets the type system help with avoiding spurious runtime errors
if forgetting to check for a sentinel. 
It makes it explicit, as a `.library!`, where code assumes that there is
a valid library.

(Some small drive-by tweaks.)

Author: lrhn

lib/resources/docs.dart.js

@@ -24,15 +24,15 @@ abstract class ElementType with CommentReferable, Nameable {
   @override
   final PackageGraph packageGraph;
   @override
-  final Library library;
+  final Library? library;
 
   final String nullabilitySuffix;
 
   ElementType._(this.type, this.library, this.packageGraph)
-      : nullabilitySuffix = type.nullabilitySuffixWithin(library);
+      : nullabilitySuffix = type.nullabilitySuffixWithin;
 
   factory ElementType.for_(
-      DartType type, Library library, PackageGraph packageGraph) {
+      DartType type, Library? library, PackageGraph packageGraph) {
     runtimeStats.incrementAccumulator('elementTypeInstantiation');
     var fElement = type.documentableElement;
     if (fElement == null ||
@@ -74,7 +74,7 @@ class UndefinedElementType extends ElementType {
       : super._();
 
   factory UndefinedElementType._from(
-      DartType type, Library library, PackageGraph packageGraph) {
+      DartType type, Library? library, PackageGraph packageGraph) {
     // [UndefinedElementType]s.
     if (type.alias != null) {
       if (type is FunctionType) {
@@ -262,7 +262,7 @@ abstract class DefinedElementType extends ElementType {
       : super._();
 
   factory DefinedElementType._from(DartType type, ModelElement modelElement,
-      Library library, PackageGraph packageGraph) {
+      Library? library, PackageGraph packageGraph) {
     if (type is! TypeAliasElement && type.alias != null) {
       // Here, `alias.element` signals that this is a type referring to an
       // alias. (`TypeAliasElement.alias.element` has different implications.
@@ -360,7 +360,7 @@ mixin Rendered implements ElementType {
 
 extension on DartType {
   /// The dartdoc nullability suffix for this type in [library].
-  String nullabilitySuffixWithin(Library library) {
+  String get nullabilitySuffixWithin {
     if (this is! VoidType && !isBottom) {
       /// If a legacy type appears inside the public interface of a Null
       /// safety library, we pretend it is nullable for the purpose of

lib/resources/docs.dart.js.map

@@ -97,7 +97,7 @@ int _packageRank(List<String> packageOrder, ModelElement element) {
   var index = packageOrder.indexOf(packageName);
   if (index == -1) return packageOrder.length * 10;
   if (packageName == 'Dart' &&
-      !_dartCoreLibraries.contains(element.library.name)) {
+      !_dartCoreLibraries.contains(element.library!.name)) {
     // Non-"core" Dart SDK libraries should be ranked slightly lower than "core"
     // Dart SDK libraries. The "core" Dart SDK libraries are the ones labeled as
     // such at <https://api.dart.dev>, which can be used in both VM and Web
@@ -149,7 +149,9 @@ extension on Nameable {
     if (self is Library) return name;
     if (self is ModelElement) {
       var library = self.canonicalLibrary ?? self.library;
-      return '${library.name}.${self.qualifiedName}';
+      if (library != null) {
+        return '${library.name}.${self.qualifiedName}';
+      }
     }
     return name;
   }