Make pkg/dart_messages strong.

BUG=
R=paulberry@google.com

Review-Url: https://codereview.chromium.org/2916023004 .

pkg/dart_messages/analysis_options.yaml

@@ -0,0 +1,2 @@
+analyzer:
+  strong-mode: true

pkg/dart_messages/bin/message_id.dart

@@ -4,7 +4,7 @@
 
 import 'dart:math' as math;
 
-import '../lib/shared_messages.dart' as shared_messages;
+import 'package:dart_messages/shared_messages.dart' as shared_messages;
 
 math.Random random = new math.Random();
 
@@ -13,7 +13,7 @@ final $A = "A".codeUnitAt(0);
 final $Z = "Z".codeUnitAt(0);
 
 String computeId() {
-  List charCodes = [];
+  List<int> charCodes = [];
   for (int i = 0; i < idLength; i++) {
     charCodes.add($A + random.nextInt($Z - $A));
   }

pkg/dart_messages/bin/publish.dart

@@ -5,7 +5,7 @@
 import 'dart:convert';
 import 'dart:io' as io;
 
-import '../lib/shared_messages.dart';
+import 'package:dart_messages/shared_messages.dart';
 
 const String jsonPath = '../lib/generated/shared_messages.json';
 const String dart2jsPath =

pkg/dart_messages/test/dart_messages_test.dart

@@ -4,7 +4,7 @@
 
 import 'dart:io' as io;
 
-import '../lib/shared_messages.dart';
+import 'package:dart_messages/shared_messages.dart';
 
 void testJsonIsUpdated() {
   var packageRoot = io.Platform.packageRoot;