[stable] [analysis_server] Fix missing closure completions in LSP

Closure completions disappeared in eb73dba because we ended up wiping our the filterText/label when trying to clean them up for functions.

Fixes Dart-Code/Dart-Code#4837

Bug: Dart-Code/Dart-Code#4837
Change-Id: I536ce38624ee3f6047229ac58c3fe173d162079a
Cherry-pick: https://dart-review.googlesource.com/c/sdk/+/335822
Cherry-pick-request: #54112
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/337341
Reviewed-by: Alexander Thomas <athom@google.com>
Commit-Queue: Alexander Thomas <athom@google.com>
Reviewed-by: Brian Wilkerson <brianwilkerson@google.com>

CHANGELOG.md

@@ -5,7 +5,11 @@ This is a patch release that:
 - Adjusts the nullablity computations in the implementation of the
   upper bound algorithm in the CFE (issue [#53999][]).
 
+- Fixes missing closure code completion entries for function parameters 
+  (issue [#54112][]) for LSP-based editors like VS Code.
+
 [#53999]: https://github.com/dart-lang/sdk/issues/53999
+[#54112]: https://github.com/dart-lang/sdk/issues/54112
 
 ## 3.2.1 - 2023-11-22
 

pkg/analysis_server/lib/src/lsp/mapping.dart

@@ -913,7 +913,13 @@ lsp.CompletionItem toCompletionItem(
   // Displayed labels may have additional info appended (for example '(...)' on
   // callables and ` => ` on getters) that should not be included in filterText,
   // so strip anything from the first paren/space.
-  final filterText = label.split(_completionFilterTextSplitPattern).first;
+  //
+  // Only do this if label doesn't start with the pattern, because if it does
+  // (for example for a closure `(a, b) {}`) we'll end up with an empty string
+  // but we should instead use the whole label.
+  final filterText = !label.startsWith(_completionFilterTextSplitPattern)
+      ? label.split(_completionFilterTextSplitPattern).first
+      : label;
 
   // If we're using label details, we also don't want the label to include any
   // additional symbols as noted above, because they will appear in the extra

pkg/analysis_server/test/lsp/completion_dart_test.dart

@@ -762,6 +762,32 @@ class B {
     expect(labels, contains('Deprecated(…)'));
   }
 
+  Future<void> test_closure() async {
+    final content = '''
+void f({void Function(int a, String b) closure}) {}
+
+void g() {
+  f(closure: ^);
+}
+''';
+
+    final expectedContent = '''
+void f({void Function(int a, String b) closure}) {}
+
+void g() {
+  f(closure: (a, b) => ^,);
+}
+''';
+
+    await verifyCompletions(
+      mainFileUri,
+      content,
+      expectCompletions: ['(a, b) {}', '(a, b) =>'],
+      applyEditsFor: '(a, b) =>',
+      expectedContent: expectedContent,
+    );
+  }
+
   Future<void> test_comment() async {
     final content = '''
     // foo ^