Use declared return type for inference of return and yield statements.

pkg/front_end/tool/fasta_perf.dart

Index: pkg/front_end/tool/fasta_perf.dart
diff --git a/pkg/front_end/tool/fasta_perf.dart b/pkg/front_end/tool/fasta_perf.dart
index d8ccbaa4efbdad9b1671acc2ae7cfa71a504cdea..4cc7d9ac79db125cee48d3a8a82a329b9751416a 100644
--- a/pkg/front_end/tool/fasta_perf.dart
+++ b/pkg/front_end/tool/fasta_perf.dart
@@ -22,6 +22,7 @@ import 'package:front_end/src/fasta/translate_uri.dart' show TranslateUri;
 import 'package:front_end/src/fasta/translate_uri.dart';
 import 'package:front_end/src/fasta/parser/dart_vm_native.dart'
     show skipNativeClause;
+import 'package:kernel/ast.dart' show FunctionNode;
 
 /// Cumulative total number of chars scanned.
 int inputSize = 0;
@@ -201,7 +202,7 @@ class _PartialAstBuilder extends AstBuilder {
 
   // Note: this method converts the body to kernel, so we skip that here.
   @override
-  finishFunction(formals, asyncModifier, body) {}
+  finishFunction(FunctionNode function, formals, asyncModifier, body) {}
 }
 
 // Invoke the fasta kernel generator for the program starting in [entryUri]