[analyzer] Remove workaround for old flow analysis bug.

The fix for dart-lang/language#1785 was
released in Dart 2.15. The analyzer's pubspec currently requires SDK
version 3.7. So the workaround is no longer needed.

Bug: dart-lang/language#1785
Change-Id: Ia18ca2c42e709cf232b3fa2fafba8f3679bb7d70
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/437780
Commit-Queue: Konstantin Shcheglov <[email protected]>
Auto-Submit: Paul Berry <[email protected]>
Reviewed-by: Konstantin Shcheglov <[email protected]>

Author: stereotype441

pkg/analyzer/lib/src/dart/resolver/function_expression_resolver.dart

@@ -24,9 +24,7 @@ class FunctionExpressionResolver {
 
   void resolve(FunctionExpressionImpl node, {required DartType contextType}) {
     var parent = node.parent;
-    // Note: `isFunctionDeclaration` must have an explicit type to work around
-    // https://github.com/dart-lang/language/issues/1785.
-    bool isFunctionDeclaration = parent is FunctionDeclaration;
+    var isFunctionDeclaration = parent is FunctionDeclaration;
     var body = node.body;
 
     if (_resolver.flowAnalysis.flow != null && !isFunctionDeclaration) {