Cleanup some toString methods (#2532)

* Some minor toString cleanup/fixes

* Address feedback

Author: parlough

lib/src/model/package_graph.dart

@@ -704,15 +704,17 @@ class PackageGraph {
 
   @override
   String toString() {
-    final divider = '=========================================================';
-    final buffer =
-        StringBuffer('PackageGraph built from ${defaultPackage.name}');
+    const divider = '=========================================================';
+    final buffer = StringBuffer('PackageGraph built from ');
+    buffer.writeln(defaultPackageName);
     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.write('Package $name documented at ${package.documentedWhere} '
+          'with libraries: ');
+      buffer.writeAll(package.allLibraries);
+      buffer.writeln();
     }
     buffer.writeln(divider);
     return buffer.toString();

lib/src/warnings.dart

@@ -520,10 +520,10 @@ class PackageWarningCounter {
 
   @override
   String toString() {
-    var errors = '$errorCount ${errorCount == 1 ? "error" : "errors"}';
-    var warnings =
+    final errors = '$errorCount ${errorCount == 1 ? "error" : "errors"}';
+    final warnings =
         '$warningCount ${warningCount == 1 ? "warning" : "warnings"}';
-    return [errors, warnings].join(', ');
+    return '$errors, $warnings';
   }
 }