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/format_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:convert';
 import 'dart:io' as io;
@@ -13,8 +15,8 @@ import 'package:quiver/iterables.dart';
 
 import 'common.dart';
 
-const String _googleFormatterUrl =
-    'https://github.com/google/google-java-format/releases/download/google-java-format-1.3/google-java-format-1.3-all-deps.jar';
+final Uri _googleFormatterUrl = Uri.https('github.com',
+    '/google/google-java-format/releases/download/google-java-format-1.3/google-java-format-1.3-all-deps.jar');
 
 /// A command to format all package code.
 class FormatCommand extends PluginCommand {
@@ -47,7 +49,7 @@ class FormatCommand extends PluginCommand {
     await _formatJava(googleFormatterPath);
     await _formatCppAndObjectiveC();
 
-    if (argResults['fail-on-change'] == true) {
+    if (getBoolArg('fail-on-change')) {
       final bool modified = await _didModifyAnything();
       if (modified) {
         throw ToolExit(1);
@@ -105,7 +107,7 @@ class FormatCommand extends PluginCommand {
     // 'ProcessException: Argument list too long'.
     final Iterable<List<String>> batches = partition(allFiles, 100);
     for (final List<String> batch in batches) {
-      await processRunner.runAndStream(argResults['clang-format'] as String,
+      await processRunner.runAndStream(getStringArg('clang-format'),
           <String>['-i', '--style=Google', ...batch],
           workingDir: packagesDir, exitOnError: true);
     }