Migration: split out a MigrationCliRunner API from MigrationCli.

This API can be used by tools that wish to perform migration but want
to provide their own command-line argument parsing.

Change-Id: Ic54e993139c803ded5ac39229ae84980db12184e
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/151080
Reviewed-by: Mike Fairhurst <[email protected]>

Author: stereotype441

pkg/nnbd_migration/bin/migrate.dart

@@ -16,7 +16,7 @@ main(List<String> args) async {
     } on FormatException catch (e) {
       cli.handleArgParsingException(e);
     }
-    await cli.run(argResults);
+    await cli.decodeCommandLineArgs(argResults)?.run();
   } on MigrationExit catch (migrationExit) {
     exitCode = migrationExit.exitCode;
   }