Stop running pub get at gclient sync time and fix build bugs

mojo/public/dart/third_party/args/lib/src/usage_exception.dart

Index: mojo/public/dart/third_party/args/lib/src/usage_exception.dart
diff --git a/mojo/public/dart/third_party/args/lib/src/usage_exception.dart b/mojo/public/dart/third_party/args/lib/src/usage_exception.dart
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index 0000000000000000000000000000000000000000..35cc7442e549b93da0e825ae660171c7361c3647
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+++ b/mojo/public/dart/third_party/args/lib/src/usage_exception.dart
@@ -0,0 +1,14 @@
+// Copyright (c) 2014, the Dart project authors.  Please see the AUTHORS file
+// 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.
+
+library args.usage_exception;
+
+class UsageException implements Exception {
+  final String message;
+  final String usage;
+
+  UsageException(this.message, this.usage);
+
+  String toString() => "$message\n\n$usage";
+}