Remove unnecessary uses of toExpression

Change-Id: I07933f647ee71c5357a599d06ce9268c0020cbd6
Reviewed-on: https://dart-review.googlesource.com/57707
Reviewed-by: Dan Rubel <danrubel@google.com>
Commit-Queue: Brian Wilkerson <brianwilkerson@google.com>

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

@@ -309,7 +309,7 @@ abstract class BodyBuilder<Expression, Statement, Arguments>
   }
 
   Expression toEffect(Object node) {
-    if (node is Generator) return toExpression(node.buildForEffect());
+    if (node is Generator) return node.buildForEffect();
     return toValue(node);
   }