Don't drop comments in collection literals with preserved newlines.

lib/src/front_end/delimited_list_builder.dart

@@ -138,6 +138,20 @@ final class DelimitedListBuilder {
     _commentsBeforeComma = CommentSequence.empty;
   }
 
+  /// Adds the contents of [lineBuilder] to this outer [DelimitedListBuilder].
+  ///
+  /// This is used when preserving newlines inside a collection literal. The
+  /// [lineBuilder] will be used for the elements that should be packed onto a
+  /// single line, and this builder is for the rows that are each on their own
+  /// line.
+  void addLineBuilder(DelimitedListBuilder lineBuilder) {
+    // Add the elements of the line to this builder.
+    add(lineBuilder.build());
+
+    // Make sure that any trailing comments on the line aren't lost.
+    _commentsBeforeComma = lineBuilder._commentsBeforeComma;
+  }
+
   /// Writes any comments appearing before [token] to the list.
   void addCommentsBefore(Token token) {
     // Handle comments between the preceding element and this one.

lib/src/front_end/piece_factory.dart

@@ -1082,7 +1082,7 @@ mixin PieceFactory {
               elements[i - 1].endToken, element.beginToken)) {
         // This element begins a new line. Add the elements on the previous
         // line to the list builder and start a new line.
-        builder.add(lineBuilder.build());
+        builder.addLineBuilder(lineBuilder);
         lineBuilder = DelimitedListBuilder(this, lineStyle);
         atLineStart = true;
       }
@@ -1097,7 +1097,8 @@ mixin PieceFactory {
       atLineStart = false;
     }
 
-    if (!atLineStart) builder.add(lineBuilder.build());
+    // Finish the last line if there is anything on it.
+    if (!atLineStart) builder.addLineBuilder(lineBuilder);
   }
 
   /// Writes a [VariablePiece] for a named or wildcard variable pattern.

pubspec.yaml

@@ -6,7 +6,7 @@ description: >-
   Provides an API and a CLI tool.
 repository: https://github.com/dart-lang/dart_style
 environment:
-  sdk: "^3.0.0"
+  sdk: "^3.4.0"
 
 dependencies:
   analyzer: '^6.5.0'

test/tall/expression/list_comment.stmt

@@ -143,4 +143,26 @@ var list = [
 >>> Space between block comment and ",".
 var list = [1,/* a */ 2 /* b */  , 3];
 <<<
-var list = [1, /* a */ 2 /* b */, 3];
+var list = [1, /* a */ 2 /* b */, 3];
+>>> Comment before comma with other comments.
+var x = [
+  1 // Comment 1.
+  ,
+  2 // Comment 2.
+];
+<<<
+var x = [
+  1, // Comment 1.
+  2, // Comment 2.
+];
+>>> Comment before comma with other comments.
+var x = [
+  1 // Comment 1.
+  ,
+  // Comment 2.
+];
+<<<
+var x = [
+  1, // Comment 1.
+  // Comment 2.
+];