move back to just supporting empty lines to represent separation betw…

…een comments

lib/src/emitter.dart

@@ -480,7 +480,7 @@ class DartEmitter extends Object
 
     if (spec.comments.isNotEmpty) {
       spec.comments
-          .map((line) => line == null ? '' : '// $line')
+          .map((line) => line.isEmpty ? '' : '// $line')
           .forEach(output.writeln);
       output.writeln();
     }

lib/src/specs/library.dart

@@ -31,7 +31,7 @@ abstract class Library
   ///
   /// Adding a `null` String to this list will insert a blank line between
   /// groups of comments.
-  BuiltList<String?> get comments;
+  BuiltList<String> get comments;
 
   /// A list of analysis issues to ignore (`ignore_for_file: ...`).
   BuiltList<String> get ignoreForFile;
@@ -61,7 +61,7 @@ abstract class LibraryBuilder
   ListBuilder<Spec> body = ListBuilder<Spec>();
   ListBuilder<Directive> directives = ListBuilder<Directive>();
 
-  ListBuilder<String?> comments = ListBuilder<String?>();
+  ListBuilder<String> comments = ListBuilder<String>();
   ListBuilder<String> ignoreForFile = ListBuilder<String>();
   String? name;
 }

test/specs/library_test.dart

@@ -31,29 +31,6 @@ void main() {
     });
 
     test('should emit a source file with multiple leading comments', () {
-      expect(
-        Library(
-          (b) => b
-            ..comments.addAll([
-              'Generated by foo!',
-              null,
-              'Avoid editing by hand.',
-            ])
-            ..body.add(
-              Class((b) => b..name = 'Foo'),
-            ),
-        ),
-        equalsDart(r'''
-          // Generated by foo!
-
-          // Avoid editing by hand.
-
-          class Foo { }
-        ''', DartEmitter(allocator: Allocator())),
-      );
-    });
-
-    test('should emit a source file with some empty leading comments', () {
       expect(
         Library(
           (b) => b
@@ -68,7 +45,7 @@ void main() {
         ),
         equalsDart(r'''
           // Generated by foo!
-          //
+
           // Avoid editing by hand.
 
           class Foo { }