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

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

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+# plugin
+
+Support for building plugins in Dart.
+
+This project defines a simple framework for defining plugins. A _plugin_ is the
+unit of extensibility for a host application.
+
+An _extension point_ represents one way of extending the host application. For
+example, a file viewer application might define an extension point to allow
+viewers for new file types to be added to the application.
+
+The host application must provide at least one extension point that plugins can
+extend. Every extension point must have a unique name associated with it, and
+should specify the type of the objects that will be accepted as extensions. In
+the example above, the extension point might require that all extensions
+implement the interface FileViewer.
+
+Any plugin can define extension points that other plugins can extend. The host
+application contributes its own extension point(s) by defining a plugin.
+
+An _extension_ is an object that a plugin associates with a particular extension
+point. To continue the example, a plugin that supports the viewing of .gif files
+would create an instance of GifFileViewer, a class that implements FileViewer.
+
+The framework in this package then connects the defined extensions with the
+defined extension points so that the separately contributed plugins can
+coordinate to accomplish the larger goal.
+
+## Features and bugs
+
+Please file feature requests and bugs at the [issue tracker][tracker].
+
+[tracker]: https://github.com/dart-lang/plugin/issues