CPLAT-4058 Provide additional --help output and better stderr

.gitignore

@@ -1,3 +1,3 @@
 .dart_tool/
-.packages/
+.packages
 pubspec.lock

lib/src/executables/dart2_upgrade.dart

@@ -14,6 +14,7 @@
 
 import 'dart:io';
 
+import 'package:args/args.dart';
 import 'package:codemod/codemod.dart';
 import 'package:path/path.dart' as p;
 
@@ -36,6 +37,14 @@ import '../ignoreable.dart';
 import '../util.dart';
 
 const _backwardsCompatFlag = '--backwards-compat';
+const _helpFlag = '--help';
+const _helpFlagAbbr = '-h';
+const _changesRequiredOutput = """
+To update your code, switch to Dart 2.1.0 and run the following commands:
+  pub global activate over_react_codemod ^1.0.1
+  pub global run over_react_codemod:dart2_upgrade --backwards-compat
+Then, review and commit the changes.
+""";
 
 void main(List<String> args) {
   // Whether or not backwards-compatibility (with Dart 1) is desired will
@@ -61,9 +70,13 @@ void main(List<String> args) {
     ),
     args: args,
     defaultYes: true,
+    additionalHelpOutput: argParser.usage,
+    changesRequiredOutput: _changesRequiredOutput,
   );
 
-  if (exitCode > 0) {
+  if (exitCode > 0 ||
+      args.contains(_helpFlag) ||
+      args.contains(_helpFlagAbbr)) {
     return;
   }
 
@@ -113,5 +126,13 @@ void main(List<String> args) {
     ],
     args: args,
     defaultYes: true,
+    changesRequiredOutput: _changesRequiredOutput,
   );
 }
+
+final argParser = ArgParser()
+  ..addFlag(
+    'backwards-compat',
+    negatable: false,
+    help: 'Maintain backwards-compatibility with Dart 1.',
+  );

pubspec.yaml

@@ -19,7 +19,7 @@ environment:
 dependencies:
   analyzer: ^0.33.6+1
   args: ^1.5.1
-  codemod: ^0.1.2
+  codemod: ^0.1.3
   path: ^1.6.2
   pub_semver: ^1.4.2
   source_span: ^1.4.1
@@ -33,4 +33,4 @@ dev_dependencies:
   test: ^1.5.1
 
 executables:
-  dart2_upgrade:
+  dart2_upgrade: