Nomenclature fix: migration diff output is not a summary.

The output the migration tool generates when the web preview is
disabled shows a complete diff of all changes the tool will make, so
it's really a misnomer to call it a "summary".

Change-Id: I8c7d9f9870bf8beffb62f1d10d36a65b66c32657
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/168381
Reviewed-by: Samuel Rawlins <srawlins@google.com>
Commit-Queue: Paul Berry <paulberry@google.com>

Author: stereotype441

pkg/nnbd_migration/lib/migration_cli.dart

@@ -808,9 +808,9 @@ When finished with the preview, hit ctrl-c to terminate this process.
       await blockUntilSignalInterrupt();
       nonNullableFix.shutdownServer();
     } else {
-      logger.stdout(ansi.emphasized('Summary of changes:'));
+      logger.stdout(ansi.emphasized('Diff of changes:'));
 
-      _displayChangeSummary(_dartFixListener);
+      _displayChangeDiff(_dartFixListener);
 
       logger.stdout('');
       logger.stdout('To apply these changes, re-run the tool with '
@@ -879,7 +879,7 @@ When finished with the preview, hit ctrl-c to terminate this process.
     }
   }
 
-  void _displayChangeSummary(DartFixListener migrationResults) {
+  void _displayChangeDiff(DartFixListener migrationResults) {
     Map<String, List<DartFixSuggestion>> fileSuggestions = {};
     for (DartFixSuggestion suggestion in migrationResults.suggestions) {
       String file = suggestion.location.file;

pkg/nnbd_migration/test/migration_cli_test.dart

@@ -999,7 +999,7 @@ int? f() => null
     assertNormalExit(cliRunner);
     // Check that a summary was printed
     var output = logger.stdoutBuffer.toString();
-    expect(output, contains('Summary'));
+    expect(output, contains('Diff of changes'));
     // And that it refers to test.dart and pubspec.yaml
     expect(output, contains('test.dart'));
     expect(output, contains('pubspec.yaml'));