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

mojo/public/dart/third_party/async/README.md

Index: mojo/public/dart/third_party/async/README.md
diff --git a/mojo/public/dart/third_party/async/README.md b/mojo/public/dart/third_party/async/README.md
new file mode 100644
index 0000000000000000000000000000000000000000..3820f0649d3bec93e68cae91f2dbd93323a26d2f
--- /dev/null
+++ b/mojo/public/dart/third_party/async/README.md
@@ -0,0 +1,28 @@
+# Async utilities package
+
+Contains tools to work with asynchronous computations.
+
+The package contains `Stream` and `Future` related functionality,
+as well as sub-libraries with different utilities.
+
+### Zipping streams
+
+The "stream_zip.dart" sub-library contains functionality
+to combine several streams of events into a single stream of tuples of events.
+
+### Results
+The "result.dart" sub-library introduces a `Result` class that can hold either
+a value or an error.
+It allows capturing an asynchronous computation which can give either a value
+or an error, into an asynchronous computation that always gives a `Result`
+value, where errors can be treated as data.
+It also allows releasing the `Result` back into an asynchronous computation.
+
+### History.
+This package is unrelated to the discontinued `async` package with version 0.1.7.
+
+## Features and bugs
+
+Please file feature requests and bugs at the [issue tracker][tracker].
+
+[tracker]: https://github.com/dart-lang/async/issues