[cfe] Show regeneration command on unittest expectation failures

Closes #43633

Change-Id: I57f39367c5bf0827a2f0cb62e2e796692e152949
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/165660
Commit-Queue: Johnni Winther <johnniwinther@google.com>
Reviewed-by: Jens Johansen <jensj@google.com>

pkg/testing/lib/src/chain.dart

@@ -354,7 +354,12 @@ class Result<O> {
 
   final List<String> logs = <String>[];
 
-  Result(this.output, this.outcome, this.error, this.trace);
+  /// If set, running the test with '-D$autoFixCommand' will automatically
+  /// update the test to match new expectations.
+  final String autoFixCommand;
+
+  Result(this.output, this.outcome, this.error, this.trace,
+      {this.autoFixCommand});
 
   Result.pass(O output) : this(output, Expectation.Pass, null, null);