Add a message for the package not found case to help users

lib/src/model/package_graph.dart

@@ -10,6 +10,7 @@ import 'package:analyzer/file_system/file_system.dart';
 import 'package:analyzer/src/generated/sdk.dart';
 import 'package:analyzer/src/generated/source.dart';
 import 'package:analyzer/src/generated/source_io.dart';
+import 'package:dartdoc/dartdoc.dart' show DartdocFailure;
 import 'package:dartdoc/src/dartdoc_options.dart';
 import 'package:dartdoc/src/logging.dart';
 import 'package:dartdoc/src/model/comment_referable.dart';
@@ -63,6 +64,10 @@ class PackageGraph with CommentReferable, Nameable {
     var libraryElement = resolvedLibrary.element;
     var packageMeta =
         packageMetaProvider.fromElement(libraryElement, config.sdkDir);
+    if (packageMeta == null) {
+      throw DartdocFailure(packageMetaProvider.getMessageForMissingPackageMeta(
+          libraryElement, config));
+    }
     var lib = Library.fromLibraryResult(
         resolvedLibrary, this, Package.fromPackageMeta(packageMeta, this));
     packageMap[packageMeta.name].libraries.add(lib);

lib/src/package_meta.dart

@@ -44,7 +44,8 @@ final PackageMetaProvider pubPackageMetaProvider = PackageMetaProvider(
         .getFile(PhysicalResourceProvider.INSTANCE.pathContext
             .absolute(Platform.resolvedExecutable))
         .parent2
-        .parent2);
+        .parent2,
+    messageForMissingPackageMeta: PubPackageMeta.messageForMissingPackageMeta);
 
 /// Sets the supported way of constructing [PackageMeta] objects.
 ///
@@ -63,15 +64,32 @@ class PackageMetaProvider {
       _fromElement;
   final PackageMeta Function(String, ResourceProvider) _fromFilename;
   final PackageMeta Function(Folder, ResourceProvider) _fromDir;
+  final String Function(LibraryElement, DartdocOptionContext)
+      _messageForMissingPackageMeta;
 
   PackageMeta fromElement(LibraryElement library, String s) =>
       _fromElement(library, s, resourceProvider);
+
   PackageMeta fromFilename(String s) => _fromFilename(s, resourceProvider);
+
   PackageMeta fromDir(Folder dir) => _fromDir(dir, resourceProvider);
 
+  String getMessageForMissingPackageMeta(
+          LibraryElement library, DartdocOptionContext optionContext) =>
+      _messageForMissingPackageMeta(library, optionContext);
+
   PackageMetaProvider(this._fromElement, this._fromFilename, this._fromDir,
       this.resourceProvider, this.defaultSdkDir,
-      {this.defaultSdk});
+      {this.defaultSdk,
+      Function(LibraryElement, DartdocOptionContext)
+          messageForMissingPackageMeta})
+      : _messageForMissingPackageMeta = messageForMissingPackageMeta ??
+            _defaultMessageForMissingPackageMeta;
+
+  static String _defaultMessageForMissingPackageMeta(
+      LibraryElement library, DartdocOptionContext optionContext) {
+    return 'Unknown package for library: ${library.source.fullName}';
+  }
 }
 
 /// Describes a single package in the context of `dartdoc`.
@@ -255,6 +273,17 @@ abstract class PubPackageMeta extends PackageMeta {
     return _packageMetaCache[pathContext.absolute(folder.path)];
   }
 
+  /// Create a helpful user error message for a case where a package can not
+  /// be found.
+  static String messageForMissingPackageMeta(
+      LibraryElement library, DartdocOptionContext optionContext) {
+    var libraryString = library.librarySource.fullName;
+    var dartOrFlutter = optionContext.flutterRoot == null ? 'dart' : 'flutter';
+    return 'Unknown package for library: $libraryString.  Consider `$dartOrFlutter pub get` and/or '
+        '`$dartOrFlutter pub global deactivate dartdoc` followed by `$dartOrFlutter pub global activate dartdoc` to fix. '
+        'Also, be sure that `$dartOrFlutter analyze` completes without errors.';
+  }
+
   @override
   String sdkType(String flutterRootPath) {
     if (flutterRootPath != null) {

test/documentation_comment_test.dart

@@ -44,13 +44,14 @@ void main() {
       sdkFolder = utils.writeMockSdkFiles(mockSdk);
 
       packageMetaProvider = PackageMetaProvider(
-        PubPackageMeta.fromElement,
-        PubPackageMeta.fromFilename,
-        PubPackageMeta.fromDir,
-        resourceProvider,
-        sdkFolder,
-        defaultSdk: mockSdk,
-      );
+          PubPackageMeta.fromElement,
+          PubPackageMeta.fromFilename,
+          PubPackageMeta.fromDir,
+          resourceProvider,
+          sdkFolder,
+          defaultSdk: mockSdk,
+          messageForMissingPackageMeta:
+              PubPackageMeta.messageForMissingPackageMeta);
       var optionSet = await DartdocOptionSet.fromOptionGenerators(
           'dartdoc', [createDartdocOptions], packageMetaProvider);
       optionSet.parseArguments([]);

test/end2end/model_test.dart

@@ -93,6 +93,16 @@ void main() {
         packageGraph.libraries.firstWhere((lib) => lib.name == 'base_class');
   });
 
+  group('PackageMeta and PackageGraph integration', () {
+    test('PackageMeta error messages generate correctly', () {
+      var message = packageGraph.packageMetaProvider
+          .getMessageForMissingPackageMeta(
+              fakeLibrary.element, packageGraph.config);
+      expect(message, contains('fake.dart'));
+      expect(message, contains('pub global activate dartdoc'));
+    });
+  });
+
   group('triple-shift', () {
     Library tripleShift;
     Class C, E, F;

test/src/utils.dart

@@ -93,6 +93,7 @@ PackageMetaProvider get testPackageMetaProvider {
     resourceProvider,
     sdkFolder,
     defaultSdk: mockSdk,
+    messageForMissingPackageMeta: PubPackageMeta.messageForMissingPackageMeta,
   );
 }