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/version_check_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 'package:file/file.dart';
@@ -135,7 +137,7 @@ class VersionCheckCommand extends PluginCommand {
                 '"publish_to: none".');
       }
       Version sourceVersion;
-      if (argResults[_againstPubFlag] as bool) {
+      if (getBoolArg(_againstPubFlag)) {
         final String packageName = pubspecFile.parent.basename;
         final PubVersionFinderResponse pubVersionFinderResponse =
             await _pubVersionFinder.getPackageVersion(package: packageName);
@@ -161,7 +163,7 @@ ${indentation}HTTP response: ${pubVersionFinderResponse.httpResponse.body}
       }
       if (sourceVersion == null) {
         String safeToIgnoreMessage;
-        if (argResults[_againstPubFlag] as bool) {
+        if (getBoolArg(_againstPubFlag)) {
           safeToIgnoreMessage =
               '${indentation}Unable to find package on pub server.';
         } else {
@@ -181,8 +183,7 @@ ${indentation}HTTP response: ${pubVersionFinderResponse.httpResponse.body}
           getAllowedNextVersions(sourceVersion, headVersion);
 
       if (!allowedNextVersions.containsKey(headVersion)) {
-        final String source =
-            (argResults[_againstPubFlag] as bool) ? 'pub' : 'master';
+        final String source = (getBoolArg(_againstPubFlag)) ? 'pub' : 'master';
         final String error = '${indentation}Incorrectly updated version.\n'
             '${indentation}HEAD: $headVersion, $source: $sourceVersion.\n'
             '${indentation}Allowed versions: $allowedNextVersions';