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

mojo/public/dart/third_party/test/lib/src/frontend/test_on.dart

+// Copyright (c) 2015, 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 test.frontend.test_on;
+
+/// An annotation indicating which platforms a test suite supports.
+///
+/// For the full syntax of [expression], see [the README][].
+///
+/// [the README]: https://github.com/dart-lang/test/#platform-selector-syntax
+class TestOn {
+  /// The expression specifying the platform.
+  final String expression;
+
+  const TestOn(this.expression);
+}