[cfe] Implement the desugaring for ListPattern

Part of #49749

Change-Id: I3d2c35b46cdc00dbeba10cb718de80120edff0ac
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/268260
Reviewed-by: Johnni Winther <[email protected]>
Commit-Queue: Chloe Stefantsova <[email protected]>

Author: chloestefantsova

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

@@ -3463,8 +3463,6 @@ class BodyBuilder extends StackListenerImpl
     Pattern? pattern = condition.pattern;
     if (pattern != null) {
       for (VariableDeclaration variable in pattern.declaredVariables) {
-        // Skip synthetic variables.
-        if (variable.name == null) continue;
         declareVariable(variable, scope);
       }
     }

pkg/front_end/lib/src/fasta/kernel/forest.dart

@@ -243,14 +243,15 @@ class Forest {
 
   Expression createAsExpression(
       int fileOffset, Expression expression, DartType type,
-      {required bool forNonNullableByDefault}) {
+      {required bool forNonNullableByDefault, bool forDynamic = false}) {
     // ignore: unnecessary_null_comparison
     assert(forNonNullableByDefault != null);
     // ignore: unnecessary_null_comparison
     assert(fileOffset != null);
     return new AsExpression(expression, type)
       ..fileOffset = fileOffset
-      ..isForNonNullableByDefault = forNonNullableByDefault;
+      ..isForNonNullableByDefault = forNonNullableByDefault
+      ..isForDynamic = forDynamic;
   }
 
   Expression createSpreadElement(int fileOffset, Expression expression,