[dart2js] Avoid capturing dangling else in labeled then-part

Bug: https://github.com/flutter/flutter/issues/122724

Change-Id: I431bd8c1147958cefd6874187364bcf7f347c966
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/289066
Reviewed-by: Nate Biggs <natebiggs@google.com>
Commit-Queue: Stephen Adams <sra@google.com>

pkg/js_ast/lib/src/printer.dart

@@ -1661,7 +1661,13 @@ class DanglingElseVisitor extends BaseVisitor<bool> {
   bool visitComment(Comment node) => true;
 
   @override
-  bool visitBlock(Block node) => false;
+  bool visitBlock(Block node) {
+    // Singleton blocks are in many places printed as the contained statement so
+    // that statement might capture the dangling else.
+    if (node.statements.length != 1) return false;
+    return node.statements.single.accept(this);
+  }
+
   @override
   bool visitExpressionStatement(ExpressionStatement node) => false;
   @override

pkg/js_ast/test/dangling_else_label_test.dart

@@ -0,0 +1,146 @@
+// Copyright (c) 2023, the Dart project authors.  Please see the AUTHORS file
+// for details. All rights reserved. Use of this source code is governed by a
+// BSD-style license that can be found in the LICENSE file.
+
+// Test for dangling-else detection when the then-part has a label. The
+// then-part of an if-then-else statement sometimes needs to be wrapped in a
+// block to avoid an inner if-then 'capturing' the else part.  Often singleton
+// blocks are printed as their contents, which can give the effect of the block
+// moving upwards to the if-statement.
+
+import 'dart:convert';
+import 'package:expect/expect.dart';
+import 'package:js_ast/js_ast.dart';
+
+void check1(String expected, Statement then) {
+  final actual = DebugPrint(
+      If(VariableUse('x'), then, ExpressionStatement(VariableUse('E'))));
+  Expect.equals(
+      expected,
+      actual,
+      '\n'
+      '\nexpected ${json.encode(expected)}'
+      '\nactual:  ${json.encode(actual)}');
+}
+
+void check(String expected, Statement then1,
+    [Statement? then2, Statement? then3, Statement? then4]) {
+  check1(expected, then1);
+  if (then2 != null) check1(expected, then2);
+  if (then3 != null) check1(expected, then3);
+  if (then4 != null) check1(expected, then4);
+}
+
+void main() {
+  final y = VariableUse('y');
+  final z = VariableUse('z');
+  final S1 = ExpressionStatement(VariableUse('S1'));
+  final S2 = ExpressionStatement(VariableUse('S2'));
+
+  check(
+    r'''
+if (x)
+  L:
+    if (y)
+      S1;
+    else
+      S2;
+else
+  E;
+''',
+    LabeledStatement('L', If(y, S1, S2)),
+    Block([LabeledStatement('L', If(y, S1, S2))]),
+    LabeledStatement('L', Block([If(y, S1, S2)])),
+  );
+
+  check(
+    r'''
+if (x) {
+  L:
+    if (y)
+      S1;
+} else
+  E;
+''',
+    LabeledStatement('L', If.noElse(y, S1)),
+    Block([LabeledStatement('L', If.noElse(y, S1))]),
+    LabeledStatement('L', Block([If.noElse(y, S1)])),
+  );
+
+  check(
+    r'''
+if (x)
+  L: {
+    if (y)
+      S1;
+    S2;
+  }
+else
+  E;
+''',
+    LabeledStatement('L', Block([If.noElse(y, S1), S2])),
+  );
+
+  check(
+    r'''
+if (x) {
+  L:
+    if (y)
+      S1;
+    else if (z)
+      S2;
+} else
+  E;
+''',
+    LabeledStatement('L', If(y, S1, If.noElse(z, S2))),
+    Block([LabeledStatement('L', If(y, S1, If.noElse(z, S2)))]),
+    LabeledStatement('L', Block([If(y, S1, If.noElse(z, S2))])),
+  );
+
+  check(
+    r'''
+if (x) {
+  L:
+    if (y)
+      S1;
+    else
+      if (z)
+        S2;
+} else
+  E;
+''',
+    LabeledStatement('L', If(y, S1, Block([If.noElse(z, S2)]))),
+  );
+
+  check(
+    r'''
+if (x) {
+  L:
+    while (y)
+      if (z)
+        S1;
+} else
+  E;
+''',
+    LabeledStatement('L', While(y, If.noElse(z, S1))),
+    Block([LabeledStatement('L', While(y, If.noElse(z, S1)))]),
+    LabeledStatement('L', Block([While(y, If.noElse(z, S1))])),
+    LabeledStatement('L', While(y, Block([If.noElse(z, S1)]))),
+  );
+
+  check(
+    r'''
+if (x) {
+  L:
+    for (;;)
+      if (z)
+        S1;
+} else
+  E;
+''',
+    LabeledStatement('L', For(null, null, null, If.noElse(z, S1))),
+    Block([LabeledStatement('L', For(null, null, null, If.noElse(z, S1)))]),
+    LabeledStatement('L', Block([For(null, null, null, If.noElse(z, S1))])),
+    LabeledStatement('L', For(null, null, null, Block([If.noElse(z, S1)]))),
+  );
+}