Experiment with tester on the flutter_tools general shard (#60490)

Use tester (disclaimer, written by me) to unit test the flutter tools general shard. Since these tests are designed to be hermetic, the single compilation/single isolate model offers a tremendous speedup over the default pub run test workflow. On previous linux shard, test execution alone took 10:30, while now it takes only 1:30.

dev/bots/test.dart

@@ -302,8 +302,6 @@ Future<void> _runToolCoverage() async {
 }
 
 Future<void> _runToolTests() async {
-  final bq.BigqueryApi bigqueryApi = await _getBigqueryApi();
-
   const String kDotShard = '.shard';
   const String kTest = 'test';
   final String toolsPath = path.join(flutterRoot, 'packages', 'flutter_tools');
@@ -321,14 +319,21 @@ Future<void> _runToolTests() async {
       final String suffix = Platform.isWindows && subshard == 'commands'
         ? 'permeable'
         : '';
-      await _pubRunTest(
-        toolsPath,
-        testPaths: <String>[path.join(kTest, '$subshard$kDotShard', suffix)],
-        tableData: bigqueryApi?.tabledata,
-        enableFlutterToolAsserts: true,
-        // Detect unit test time regressions (poor time delay handling, etc).
-        perTestTimeout: (subshard == 'general') ? const Duration(seconds: 2) : null,
-      );
+      // Try out tester on unit test shard
+      if (subshard == 'general') {
+        await _pubRunTester(
+          toolsPath,
+          testPaths: <String>[path.join(kTest, '$subshard$kDotShard', suffix)],
+          // Detect unit test time regressions (poor time delay handling, etc).
+          perTestTimeout: (subshard == 'general') ? 15 : null,
+        );
+      } else {
+        await _pubRunTest(
+          toolsPath,
+          testPaths: <String>[path.join(kTest, '$subshard$kDotShard', suffix)],
+          enableFlutterToolAsserts: true,
+        );
+      }
     },
   );
 
@@ -925,6 +930,67 @@ Future<void> _runFlutterWebTest(String workingDirectory, List<String> tests) asy
   );
 }
 
+const String _supportedTesterVersion = '0.0.1-dev9';
+
+Future<void> _pubRunTester(String workingDirectory, {
+  List<String> testPaths,
+  bool forceSingleCore = false,
+  int perTestTimeout,
+}) async {
+  int cpus;
+  final String cpuVariable = Platform.environment['CPU']; // CPU is set in cirrus.yml
+  if (cpuVariable != null) {
+    cpus = int.tryParse(cpuVariable, radix: 10);
+    if (cpus == null) {
+      print('${red}The CPU environment variable, if set, must be set to the integer number of available cores.$reset');
+      print('Actual value: "$cpuVariable"');
+      exit(1);
+    }
+  } else {
+    cpus = 2; // Don't default to 1, otherwise we won't catch race conditions.
+  }
+  // Integration tests that depend on external processes like chrome
+  // can get stuck if there are multiple instances running at once.
+  if (forceSingleCore) {
+    cpus = 1;
+  }
+  final List<String> args = <String>[
+    'global',
+    'activate',
+    'tester',
+    _supportedTesterVersion
+  ];
+  final Map<String, String> pubEnvironment = <String, String>{
+    'FLUTTER_ROOT': flutterRoot,
+  };
+  if (Directory(pubCache).existsSync()) {
+    pubEnvironment['PUB_CACHE'] = pubCache;
+  }
+  await runCommand(
+    pub,
+    args,
+    workingDirectory: workingDirectory,
+    environment: pubEnvironment,
+  );
+  await runCommand(
+    pub,
+    <String>[
+      'global',
+      'run',
+      'tester',
+      '-j$cpus',
+      '--ci',
+      if (perTestTimeout != null)
+        '--timeout=$perTestTimeout'
+      else
+        '--timeout=-1',
+      ...testPaths,
+    ],
+    workingDirectory: workingDirectory,
+    environment: pubEnvironment,
+  );
+}
+
 Future<void> _pubRunTest(String workingDirectory, {
   List<String> testPaths,
   bool enableFlutterToolAsserts = true,