Add support for sharding test runs.

lib/src/runner/configuration/args.dart

 // Copyright (c) 2016, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 import 'dart:io';
 
 import 'package:args/args.dart';
 import 'package:boolean_selector/boolean_selector.dart';
 
 import '../../backend/test_platform.dart';
@@ skipping 67 matching lines @@
       valueHelp: 'port');
   parser.addOption("timeout",
       help: 'The default test timeout. For example: 15s, 2x, none',
       defaultsTo: '30s');
   parser.addFlag("pause-after-load",
       help: 'Pauses for debugging before any tests execute.\n'
           'Implies --concurrency=1 and --timeout=none.\n'
           'Currently only supported for browser tests.',
       negatable: false);
 
+  // These are used by the internal Google test runner, so they're hidden from
+  // the --help output but still supported as stable API surface. See
+  // [Configuration.shardIndex] for details on their semantics.
+  parser.addOption("shard-index", hide: true);
+  parser.addOption("total-shards", hide: true);
+
   parser.addSeparator("======== Output");
   parser.addOption("reporter",
       abbr: 'r',
       help: 'The runner used to print test results.',
       defaultsTo: defaultReporter,
       allowed: allReporters,
       allowedHelp: {
     'compact': 'A single line, updated continuously.',
     'expanded': 'A separate line for each update.',
     'json': 'A machine-readable format (see https://goo.gl/0HRhdZ).'
@@ skipping 41 matching lines @@
     });
 
     var excludeTagSet = new Set.from(_options['exclude-tags'] ?? [])
       ..addAll(_options['exclude-tag'] ?? []);
 
     var excludeTags = excludeTagSet.fold(BooleanSelector.none, (selector, tag) {
       var tagSelector = new BooleanSelector.parse(tag);
       return selector.union(tagSelector);
     });
 
+    var shardIndex = _parseOption('shard-index', int.parse);
+    var totalShards = _parseOption('total-shards', int.parse);
+    if ((shardIndex == null) != (totalShards == null)) {
+      throw new FormatException(
+          "--shard-index and --total-shards may only be passed together.");
+    } else if (shardIndex != null) {
+      if (shardIndex < 0) {
+        throw new FormatException("--shard-index may not be negative.");
+      } else if (shardIndex >= totalShards) {
+        throw new FormatException(
+            "--shard-index must be less than --total-shards.");
+      }
+    }
+
     return new Configuration(
         help: _ifParsed('help'),
         version: _ifParsed('version'),
         verboseTrace: _ifParsed('verbose-trace'),
         jsTrace: _ifParsed('js-trace'),
         pauseAfterLoad: _ifParsed('pause-after-load'),
         color: _ifParsed('color'),
         packageRoot: _ifParsed('package-root'),
         reporter: _ifParsed('reporter'),
         pubServePort: _parseOption('pub-serve', int.parse),
         concurrency: _parseOption('concurrency', int.parse),
+        shardIndex: shardIndex,
+        totalShards: totalShards,
         timeout: _parseOption('timeout', (value) => new Timeout.parse(value)),
         patterns: patterns,
         platforms: _ifParsed('platform')?.map(TestPlatform.find),
         chosenPresets: _ifParsed('preset'),
         paths: _options.rest.isEmpty ? null : _options.rest,
         includeTags: includeTags,
         excludeTags: excludeTags);
   }
 
   /// Returns the parsed option for [name], or `null` if none was parsed.
@@ skipping 18 matching lines @@
   /// information.
   _wrapFormatException(String name, parse()) {
     try {
       return parse();
     } on FormatException catch (error) {
       throw new FormatException('Couldn\'t parse --$name "${_options[name]}": '
           '${error.message}');
     }
   }
 }