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/build_examples_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;
 
@@ -52,7 +54,7 @@ class BuildExamplesCommand extends PluginCommand {
     ];
     final Map<String, bool> platforms = <String, bool>{
       for (final String platform in platformSwitches)
-        platform: argResults[platform] as bool
+        platform: getBoolArg(platform)
     };
     if (!platforms.values.any((bool enabled) => enabled)) {
       print(
@@ -63,7 +65,7 @@ class BuildExamplesCommand extends PluginCommand {
     final String flutterCommand =
         const LocalPlatform().isWindows ? 'flutter.bat' : 'flutter';
 
-    final String enableExperiment = argResults[kEnableExperiment] as String;
+    final String enableExperiment = getStringArg(kEnableExperiment);
 
     final List<String> failingPackages = <String>[];
     await for (final Directory plugin in getPlugins()) {