Preserve one blank line between elements in delimited lists. (#1483)

This is consistent with the old formatter and lets users break groups
of elements into "sections" inside basically all delimited constructs:
lists, maps, objects, argument lists, enums, etc.

Author: munificent

lib/src/front_end/delimited_list_builder.dart

@@ -207,7 +207,11 @@ class DelimitedListBuilder {
   /// before the closing delimiter.
   void _addComments(CommentSequence comments, {required bool hasElementAfter}) {
     // Early out if there's nothing to do.
-    if (_commentsBeforeComma.isEmpty && comments.isEmpty) return;
+    if (_commentsBeforeComma.isEmpty &&
+        comments.isEmpty &&
+        comments.linesBeforeNextToken <= 1) {
+      return;
+    }
 
     if (_commentsBeforeComma.requiresNewline || comments.requiresNewline) {
       _mustSplit = true;
@@ -238,6 +242,11 @@ class DelimitedListBuilder {
       }
     }
 
+    // Preserve one blank line between successive elements.
+    if (_elements.isNotEmpty && comments.linesBeforeNextToken > 1) {
+      _blanksAfter.add(_elements.last);
+    }
+
     // Comments that are neither hanging nor leading are treated like their own
     // elements.
     for (var i = 0; i < separateComments.length; i++) {

test/tall/declaration/enum.unit

@@ -130,4 +130,46 @@ enum SomeEnumType<
   a,
   b,
   c,
-}
+}
+>>> Remove blank lines before first and last value. Preserve one between.
+enum E {
+
+
+  firstConstant,
+
+
+
+  secondConstant,
+
+
+
+  thirdConstant
+
+
+}
+<<<
+enum E {
+  firstConstant,
+
+  secondConstant,
+
+  thirdConstant,
+}
+>>> Discard blank lines if doesn't need to split.
+enum E {
+
+
+  a,
+
+
+
+  b,
+
+
+
+  c,
+
+
+}
+<<<
+enum E { a, b, c }

test/tall/declaration/enum_comment.unit

@@ -63,6 +63,7 @@ enum A {
 <<<
 enum A {
   B,
+
   // comment
 }
 >>> Ensure blank line above doc comments.

test/tall/declaration/enum_metadata.unit

@@ -26,6 +26,7 @@ enum Foo { a,
 <<<
 enum Foo {
   a,
+
   @meta
   b,
 }

test/tall/expression/list.stmt

@@ -47,7 +47,7 @@ const [1, 2, 3];
       fourth +
       fifth,
 ];
->>> Remove blank lines around elements.
+>>> Remove blank lines before first and last elements. Preserve one between.
 [
 
 
@@ -66,9 +66,29 @@ const [1, 2, 3];
 <<<
 [
   firstElement,
+
   secondElement,
+
   thirdElement,
 ];
+>>> Discard blank lines if doesn't need to split.
+[
+
+
+  1,
+
+
+
+  2,
+
+
+
+  3,
+
+
+];
+<<<
+[1, 2, 3];
 >>> With type argument.
 <  int  >  [  1  ,  2  ,  3  ];
 <<<

test/tall/expression/list_comment.stmt

@@ -106,6 +106,7 @@ var list = [
 [
   // comment
   element,
+
   noComment,
 
   // comment

test/tall/expression/list_preserve_newlines.stmt

@@ -73,6 +73,7 @@ var list = [
   element3,
   element4,
   element5,
+
   element6, element7,
 
   // comment

test/tall/expression/map.stmt

@@ -77,7 +77,7 @@ const {first: 1, second: 2};
       fifth +
       sixth,
 });
->>> Remove blank lines around entries.
+>>> Remove blank lines before first and last entries. Preserve one between.
 ({
 
 
@@ -96,9 +96,29 @@ const {first: 1, second: 2};
 <<<
 ({
   firstElement: 1,
+
   secondElement: 2,
+
   thirdElement: 3,
 });
+>>> Discard blank lines if doesn't need to split.
+({
+
+
+  1,
+
+
+
+  2,
+
+
+
+  3,
+
+
+});
+<<<
+({1, 2, 3});
 >>> With type arguments.
 <  int  ,  String  >  {  1  :  'one'  ,  2  :  'two'  };
 <<<

test/tall/expression/map_comment.stmt

@@ -52,6 +52,7 @@ var map = {
 ({
   // comment
   element: 1,
+
   noComment: 2,
 
   // comment

test/tall/expression/map_preserve_newlines.stmt

@@ -42,6 +42,7 @@ var map = {
   element3: 3,
   element4: 4,
   element5: 5,
+
   element6: 6, element7: 7,
 
   // comment