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

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

+## 1.1.2
+
+* Export `UnmodifiableMapView` from the Dart core libraries.
+
+## 1.1.1
+
+* Bug-fix for signatures of `isValidKey` arguments of `CanonicalizedMap`.
+
+## 1.1.0
+
+* Add a `QueueList` class that implements both `Queue` and `List`.
+
+## 0.9.4
+
+* Add a `CanonicalizedMap` class that canonicalizes its keys to provide a custom
+  equality relation.
+
+## 0.9.3+1
+
+* Fix all analyzer hints.
+
+## 0.9.3
+
+* Add a `MapKeySet` class that exposes an unmodifiable `Set` view of a `Map`'s
+  keys.
+
+* Add a `MapValueSet` class that takes a function from values to keys and uses
+  it to expose a `Set` view of a `Map`'s values.