Fix a few analyzer warnings

R=nweiz@google.com

Review URL: https://codereview.chromium.org//2285133005 .

lib/src/runner/browser/post_message_channel.dart

@@ -2,7 +2,6 @@
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
-import 'dart:convert';
 import 'dart:html';
 
 import 'package:stream_channel/stream_channel.dart';

test/io.dart

@@ -8,7 +8,6 @@ library test.test.io;
 
 import 'dart:async';
 import 'dart:io';
-import 'dart:isolate';
 
 import 'package:package_resolver/package_resolver.dart';
 import 'package:path/path.dart' as p;
@@ -134,12 +133,11 @@ ScheduledProcess runTest(List<String> args, {String reporter,
 /// Runs Dart.
 ScheduledProcess runDart(List<String> args, {Map<String, String> environment,
     String description}) {
-  var allArgs = Platform.executableArguments
-      .where((arg) =>
-          !arg.startsWith("--package-root=") && !arg.startsWith("--packages="))
-      .toList()
-      ..add(PackageResolver.current.processArgument)
-      ..addAll(args);
+  var allArgs = <Object>[]
+    ..addAll(Platform.executableArguments.where((arg) =>
+        !arg.startsWith("--package-root=") && !arg.startsWith("--packages=")))
+    ..add(PackageResolver.current.processArgument)
+    ..addAll(args);
 
   return new ScheduledProcess.start(
       p.absolute(Platform.resolvedExecutable), allArgs,