Begin migrating tools to NNBD (#3891)

- Updates dependencies to null-safe versions
- Migrates common.dart (which doesn't depend on anything)
- Migrates common_tests.dart and its one dependency, utils.dart
- Adds build_runner for Mockito mock generation
- Adds a new utility methods for getting arguments that handle both the casting and the removal of nullability to address a common problematic pattern while migrating code.
  - Converts all files, not just the migrated ones, to those new helpers.

Migrating common.dart and utils.dart should unblock a command-by-command migration to null safety.

Reverts the separate of podspect lints into a step that doesn't do a Flutter upgrade
(https://github.com/flutter/plugins/pull/3700) because without that step we had a
version of Dart too old to run null-safe tooling.

First step of https://github.com/flutter/flutter/issues/81912

script/tool/lib/src/firebase_test_lab_command.dart

@@ -2,6 +2,8 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
+// @dart=2.9
+
 import 'dart:async';
 import 'dart:io' as io;
 
@@ -30,7 +32,7 @@ class FirebaseTestLabCommand extends PluginCommand {
         defaultsTo:
             p.join(io.Platform.environment['HOME'], 'gcloud-service-key.json'));
     argParser.addOption('test-run-id',
-        defaultsTo: Uuid().v4(),
+        defaultsTo: const Uuid().v4(),
         help:
             'Optional string to append to the results path, to avoid conflicts. '
             'Randomly chosen on each invocation if none is provided. '
@@ -78,7 +80,7 @@ class FirebaseTestLabCommand extends PluginCommand {
       <String>[
         'auth',
         'activate-service-account',
-        '--key-file=${argResults['service-key']}',
+        '--key-file=${getStringArg('service-key')}',
       ],
       exitOnError: true,
       logOnError: true,
@@ -87,7 +89,7 @@ class FirebaseTestLabCommand extends PluginCommand {
       'config',
       'set',
       'project',
-      argResults['project'] as String,
+      getStringArg('project'),
     ]);
     if (exitCode == 0) {
       _print('\nFirebase project configured.');
@@ -125,7 +127,7 @@ class FirebaseTestLabCommand extends PluginCommand {
       final Directory androidDirectory =
           fileSystem.directory(p.join(exampleDirectory.path, 'android'));
 
-      final String enableExperiment = argResults[kEnableExperiment] as String;
+      final String enableExperiment = getStringArg(kEnableExperiment);
       final String encodedEnableExperiment =
           Uri.encodeComponent('--enable-experiment=$enableExperiment');
 
@@ -213,7 +215,7 @@ class FirebaseTestLabCommand extends PluginCommand {
             continue;
           }
           final String buildId = io.Platform.environment['CIRRUS_BUILD_ID'];
-          final String testRunId = argResults['test-run-id'] as String;
+          final String testRunId = getStringArg('test-run-id');
           final String resultsDir =
               'plugins_android_test/$packageName/$buildId/$testRunId/${resultsCounter++}/';
           final List<String> args = <String>[
@@ -229,10 +231,10 @@ class FirebaseTestLabCommand extends PluginCommand {
             'build/app/outputs/apk/androidTest/debug/app-debug-androidTest.apk',
             '--timeout',
             '5m',
-            '--results-bucket=${argResults['results-bucket']}',
+            '--results-bucket=${getStringArg('results-bucket')}',
             '--results-dir=$resultsDir',
           ];
-          for (final String device in argResults['device'] as List<String>) {
+          for (final String device in getStringListArg('device')) {
             args.addAll(<String>['--device', device]);
           }
           exitCode = await processRunner.runAndStream('gcloud', args,