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

mojo/public/dart/third_party/yaml/lib/src/yaml_exception.dart

Index: mojo/public/dart/third_party/yaml/lib/src/yaml_exception.dart
diff --git a/mojo/public/dart/third_party/yaml/lib/src/yaml_exception.dart b/mojo/public/dart/third_party/yaml/lib/src/yaml_exception.dart
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index 0000000000000000000000000000000000000000..86899531e822c98c62f4fac201f422c17954571d
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+++ b/mojo/public/dart/third_party/yaml/lib/src/yaml_exception.dart
@@ -0,0 +1,14 @@
+// Copyright (c) 2013, 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 yaml.exception;
+
+import 'package:source_span/source_span.dart';
+
+/// An error thrown by the YAML processor.
+class YamlException extends SourceSpanFormatException {
+  YamlException(String message, SourceSpan span)
+      : super(message, span);
+}
+