Infer return types of closures (basic support).

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

Index: pkg/front_end/lib/src/fasta/kernel/kernel_shadow_ast.dart
diff --git a/pkg/front_end/lib/src/fasta/kernel/kernel_shadow_ast.dart b/pkg/front_end/lib/src/fasta/kernel/kernel_shadow_ast.dart
index 937242b4458a1f7cc9e2a0d77be297cda6e94f50..c613219a77babb153d6842a1c0734fd62596db8f 100644
--- a/pkg/front_end/lib/src/fasta/kernel/kernel_shadow_ast.dart
+++ b/pkg/front_end/lib/src/fasta/kernel/kernel_shadow_ast.dart
@@ -517,11 +517,11 @@ class KernelRethrow extends Rethrow implements KernelExpression {
 
 /// Concrete shadow object representing a return statement in kernel form.
 class KernelReturnStatement extends ReturnStatement implements KernelStatement {
-  KernelReturnStatement([KernelExpression expression]) : super(expression);
+  KernelReturnStatement([Expression expression]) : super(expression);
 
   @override
   void _inferStatement(KernelTypeInferrer inferrer) {
-    // TODO(paulberry): implement.
+    inferrer.inferReturnStatement(expression);
   }
 }