Contribute functionality that helps debugging (#2203)

Author: mehmetf

bin/dartdoc.dart

@@ -98,6 +98,9 @@ Future<void> main(List<String> arguments) async {
     }, onError: (e, Chain chain) {
       if (e is DartdocFailure) {
         stderr.writeln('\ndartdoc failed: ${e}.');
+        if (config.verboseWarnings) {
+          stderr.writeln(chain.terse);
+        }
         exitCode = 1;
         return;
       } else {

lib/src/model/package_graph.dart

@@ -650,7 +650,20 @@ class PackageGraph {
   }
 
   @override
-  String toString() => 'PackageGraph built from ${defaultPackage.name}';
+  String toString() {
+    final divider = '=========================================================';
+    final buffer =
+        StringBuffer('PackageGraph built from ${defaultPackage.name}');
+    buffer.writeln(divider);
+    buffer.writeln();
+    for (final name in packageMap.keys) {
+      final package = packageMap[name];
+      buffer.writeln('Package $name documented at ${package.documentedWhere} '
+          'with libraries: ${package.allLibraries}');
+    }
+    buffer.writeln(divider);
+    return buffer.toString();
+  }
 
   final Map<Element, Library> _canonicalLibraryFor = {};