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

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

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+## 1.3.0
+
+- Added `StreamCompleter` class for creating a stream now and providing its
+  events later as another stream.
+
+- Added `StreamQueue` class which allows requesting events from a stream
+  before they are avilable. It is like a `StreamIterator` that can queue
+  requests.
+
+- Added `SubscriptionStream` which creates a single-subscription stream
+  from an existing stream subscription.
+
+- Added a `ResultFuture` class for synchronously accessing the result of a
+  wrapped future.
+
+- Added `FutureGroup.onIdle` and `FutureGroup.isIdle`, which provide visibility
+  into whether a group is actively waiting on any futures.
+
+- Add an `AsyncMemoizer` class for running an asynchronous block of code exactly
+  once.
+
+- Added delegating wrapper classes for a number of core async types:
+  `DelegatingFuture`, `DelegatingStreamConsumer`, `DelegatingStreamController`,
+  `DelegatingSink`, `DelegatingEventSink`, `DelegatingStreamSink`, and
+  `DelegatingStreamSubscription`. These are all simple wrappers that forward all
+  calls to the wrapped objects. They can be used to expose only the desired
+  interface for subclasses, or extended to add extra functionality.
+
+## 1.2.0
+
+- Added a `FutureGroup` class for waiting for a group of futures, potentially of
+  unknown size, to complete.
+
+- Added a `StreamGroup` class for merging the events of a group of streams,
+  potentially of unknown size.
+
+## 1.1.1
+
+- Updated SDK version constraint to at least 1.9.0.
+
+## 1.1.0
+
+- ChangeLog starts here.