Dart 2 may add a restriction that void values cannot be returned in a function that returns dynamic. dart-lang/sdk#32161

This commonly appears in the form `f() => g()` where `g()` returns void, and `f()` is a top-level or a method, so it gets an implicit dynamic rather than type inference. Even if the restriction is not added to dart 2, this migration creates cleaner code that more often expresses the true intent.

#codehealth

Tested:
    TAP --sample for global presubmit queue
    http://test/OCL:186244698:BASE:186239924:1519083758984:829050d9
PiperOrigin-RevId: 186623288

Author: MichaelRFairhurst

_tests/test/core/linker/integration_dart_test.dart

@@ -164,5 +164,5 @@ class DirectiveLoggingChecks implements DoCheck {
   DirectiveLoggingChecks(this.log);
 
   @override
-  ngDoCheck() => log.info("check");
+  void ngDoCheck() => log.info("check");
 }