[cfe] Introduce InternalSyntheticVariable internal AST node

The new node is generated in for-in loops with synthetic variables.

Part of https://github.com/dart-lang/sdk/issues/61572

Change-Id: Icaa63097a145d07cdcfe21edc0487a47f90da8dd
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/487060
Commit-Queue: Chloe Stefantsova <cstefantsova@google.com>
Reviewed-by: Johnni Winther <johnniwinther@google.com>

pkg/front_end/lib/src/kernel/body_builder.dart

@@ -8236,13 +8236,22 @@ class BodyBuilderImpl extends StackListenerImpl
         lvalue.isConst = false;
       }
     } else {
-      VariableDeclaration variable = elements.syntheticVariableDeclaration =
-          forest.createVariableDeclaration(
-            offsetForToken(forToken),
-            null,
-            isFinal: true,
-            isSynthesized: true,
-          );
+      ExpressionVariable astVariable = isClosureContextLoweringEnabled
+          ? new SyntheticVariable(type: const DynamicType())
+          : forest.createVariableDeclaration(
+              offsetForToken(forToken),
+              null,
+              isFinal: true,
+              isSynthesized: true,
+            );
+
+      ExpressionVariable variable = elements.syntheticVariableDeclaration =
+          isClosureContextLoweringEnabled
+          ? new InternalSyntheticVariable(
+              astVariable: astVariable as SyntheticVariable,
+              isImplicitlyTyped: false,
+            )
+          : astVariable;
       if (lvalue is Generator) {
         /// We are in this case, where `lvalue` isn't a [VariableDeclaration]:
         ///
@@ -8255,7 +8264,7 @@ class BodyBuilderImpl extends StackListenerImpl
         ///       body;
         ///     }
         elements.syntheticAssignment = lvalue.buildAssignment(
-          new VariableGet(variable)..fileOffset = inToken.offset,
+          new VariableGet(astVariable)..fileOffset = inToken.offset,
           voidContext: true,
         );
       } else if (lvalue is Pattern) {

pkg/front_end/lib/src/kernel/internal_ast.dart

@@ -1334,6 +1334,48 @@ class InternalCatchVariable extends TreeNode
   String get catchVariableName => astVariable.catchVariableName;
 }
 
+class InternalSyntheticVariable extends TreeNode
+    with InternalExpressionVariableMixin, DelegatingVariableMixin
+    implements SyntheticVariable, InternalExpressionVariable {
+  @override
+  SyntheticVariable astVariable;
+
+  @override
+  final bool forSyntheticToken;
+
+  @override
+  final bool isImplicitlyTyped;
+
+  @override
+  final bool isLocalFunction;
+
+  InternalSyntheticVariable({
+    required this.astVariable,
+    required this.isImplicitlyTyped,
+    this.forSyntheticToken = false,
+    this.isLocalFunction = false,
+  });
+
+  @override
+  String toString() {
+    return "InternalSyntheticVariable(${toStringInternal()})";
+  }
+
+  @override
+  // Coverage-ignore(suite): Not run.
+  void toTextInternal(AstPrinter printer) {
+    printer.writeExpressionVariable(astVariable);
+    List<String> modifiers = [
+      if (forSyntheticToken) "forSyntheticToken",
+      if (isImplicitlyTyped) "isImplicitlyTyped",
+      if (isLocalFunction) "isLocalFunction",
+    ];
+    if (modifiers.isNotEmpty) {
+      printer.write("[${modifiers.join(",")}]");
+    }
+  }
+}
+
 mixin DelegatingVariableMixin on InternalExpressionVariableMixin
     implements InternalExpressionVariable {
   @override

pkg/front_end/testcases/closure_context_lowering/synthetic_variables.dart

@@ -0,0 +1,13 @@
+// Copyright (c) 2026, 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(List<String> list) {
+  String s = "";
+  for (s in list) {
+    if (s.isNotEmpty) {
+      return s;
+    }
+  }
+  return s;
+}

pkg/front_end/testcases/closure_context_lowering/synthetic_variables.dart.strong.expect

@@ -0,0 +1,18 @@
+library;
+import self as self;
+import "dart:core" as core;
+
+static method test(positional-parameter list) → dynamic/* scope=[
+  #ctx1: not-captured VariableContext([
+    positional-parameter list;
+  ]),
+] */ {
+  s := "";
+  for (synthetic-variable #t1 in list) {
+    s = #t1;
+    if(s.{core::String::isNotEmpty}{core::bool}) {
+      return s;
+    }
+  }
+  return s;
+}

pkg/front_end/testcases/closure_context_lowering/synthetic_variables.dart.strong.modular.expect

@@ -0,0 +1,18 @@
+library;
+import self as self;
+import "dart:core" as core;
+
+static method test(positional-parameter list) → dynamic/* scope=[
+  #ctx1: not-captured VariableContext([
+    positional-parameter list;
+  ]),
+] */ {
+  s := "";
+  for (synthetic-variable #t1 in list) {
+    s = #t1;
+    if(s.{core::String::isNotEmpty}{core::bool}) {
+      return s;
+    }
+  }
+  return s;
+}

pkg/front_end/testcases/closure_context_lowering/synthetic_variables.dart.strong.outline.expect

@@ -0,0 +1,5 @@
+library;
+import self as self;
+
+static method test(positional-parameter list) → dynamic
+  ;

pkg/front_end/testcases/closure_context_lowering/synthetic_variables.dart.textual_outline.expect

@@ -0,0 +1 @@
+test(List<String> list) {}

pkg/front_end/testcases/closure_context_lowering/synthetic_variables.dart.textual_outline_modelled.expect

@@ -0,0 +1 @@
+test(List<String> list) {}

pkg/front_end/testcases/modular.status

@@ -30,3 +30,4 @@ closure_context_lowering/foo48: Crash
 closure_context_lowering/assert_captured_variables: Crash
 closure_context_lowering/late_variable_initializers: Crash
 closure_context_lowering/catch_variables: Crash
+closure_context_lowering/synthetic_variables: Crash

pkg/front_end/testcases/outline.status

@@ -27,3 +27,4 @@ closure_context_lowering/foo45: ExpectationFileMismatchSerialized
 closure_context_lowering/foo48: ExpectationFileMismatchSerialized
 closure_context_lowering/assert_captured_variables: ExpectationFileMismatchSerialized
 closure_context_lowering/late_variable_initializers: ExpectationFileMismatchSerialized
+closure_context_lowering/synthetic_variables: ExpectationFileMismatchSerialized

pkg/front_end/testcases/strong.status

@@ -268,3 +268,4 @@ closure_context_lowering/foo48: Crash
 closure_context_lowering/assert_captured_variables: Crash
 closure_context_lowering/late_variable_initializers: Crash
 closure_context_lowering/catch_variables: Crash
+closure_context_lowering/synthetic_variables: Crash

pkg/kernel/lib/src/ast/variables.dart

@@ -1543,9 +1543,7 @@ class SyntheticVariable extends ExpressionVariable {
   }
 
   @override
-  bool get isSynthesized {
-    throw new UnsupportedError("${this.runtimeType}");
-  }
+  bool get isSynthesized => true;
 
   @override
   void set isSynthesized(bool value) {
@@ -1553,9 +1551,7 @@ class SyntheticVariable extends ExpressionVariable {
   }
 
   @override
-  bool get isConst {
-    throw new UnsupportedError("${this.runtimeType}");
-  }
+  bool get isConst => false;
 
   @override
   void set isConst(bool value) {
@@ -1624,7 +1620,7 @@ class SyntheticVariable extends ExpressionVariable {
   bool get hasIsFinal => true;
 
   @override
-  bool get hasIsConst => false;
+  bool get hasIsConst => true;
 
   @override
   bool get hasIsLate => false;
@@ -1633,7 +1629,7 @@ class SyntheticVariable extends ExpressionVariable {
   bool get hasIsInitializingFormal => false;
 
   @override
-  bool get hasIsSynthesized => false;
+  bool get hasIsSynthesized => true;
 
   @override
   bool get hasIsHoisted => true;

pkg/kernel/lib/text/ast_to_text.dart

@@ -138,7 +138,7 @@ String componentToString(Component node) {
 
 class NameSystem {
   final Namer<ExpressionVariable> variables =
-      new NormalNamer<VariableDeclaration>('#t');
+      new NormalNamer<ExpressionVariable>('#t');
   final Namer<Reference> libraries = new NormalNamer<Reference>('#lib');
   final Namer<TypeParameter> typeParameters =
       new NormalNamer<TypeParameter>('#T');