Pass in a language version when calling the formatter.

There is an upcoming version of dart_style that makes this argument
required, so every DartFormatter() constructor callsite needs to pass
it in. These are the last two I could find in the Dart SDK.

I'm not familiar with the surrounding code, but I believe passing in
DartFormatter.latestLanguage version will do the right thing:

* For the benchmark script, that's generated code that we will always
  want to be valid on the latest version of Dart.

* For the text_outline_suite, my understanding is that CFE tests that
  require older language versions always use the "// @Dart=x.y" comment
  to opt in to that version. Those comments are also respected by the
  formatter and override the languageVersion argument passed to the
  constructor. So I think passing in latestSupportedVersion will do the
  right thing.

Bug: #56687

Change-Id: If29188df479c7b41beb68ac889d56be13a395a16
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/389622
Commit-Queue: Johnni Winther <johnniwinther@google.com>
Auto-Submit: Bob Nystrom <rnystrom@google.com>
Reviewed-by: Johnni Winther <johnniwinther@google.com>

pkg/front_end/benchmarks/patterns/util.dart

@@ -241,7 +241,9 @@ class SeriesSet {
       sb.write(']),');
     }
     sb.write(']);');
-    return new DartFormatter().format(sb.toString());
+    return new DartFormatter(
+            languageVersion: DartFormatter.latestLanguageVersion)
+        .format(sb.toString());
   }
 }
 

pkg/front_end/test/fasta/textual_outline_suite.dart

@@ -158,7 +158,13 @@ class TextualOutline extends Step<TestDescription, TestDescription, Context> {
               experimentFlags.add(entry.key.name);
             }
           }
-          result = new DartFormatter(experimentFlags: experimentFlags)
+
+          // Default to the latest language version. If the test should be at
+          // an older language version, it will contain a `// @dart=x.y`
+          // comment, which takes precedence over this argument.
+          result = new DartFormatter(
+                  languageVersion: DartFormatter.latestLanguageVersion,
+                  experimentFlags: experimentFlags)
               .format(result);
         } catch (e, st) {
           formatterException = e;