Fix warnings and hints in the SDK

sdk/lib/async/stream_impl.dart

 // Copyright (c) 2012, 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.
 
 part of dart.async;
 
 /** Abstract and private interface for a place to put events. */
 abstract class _EventSink<T> {
   void _add(T data);
   void _addError(Object error, StackTrace stackTrace);
@@ skipping 999 matching lines @@
         case _STATE_EXTRA_ERROR:
           AsyncError prefetch = _futureOrPrefetch;
           _clear();
           return new _Future<bool>.immediateError(prefetch.error,
                                                   prefetch.stackTrace);
         case _STATE_EXTRA_DONE:
           _clear();
           return new _Future<bool>.immediate(false);
       }
     }
+
+    return null;

[Søren Gjesse, 2015/03/10 14:06:20]

Please don't add this. It gives the impression that getting here could be
intentional, but it is actually unreachable.

We should not put in code to suppress warnings.

How about:

Remove the else (not needed as the if above returns) and adding a default to the
switch with 'throw "Unreachable";'

   }
 
   /** Clears up the internal state when the iterator ends. */
   void _clear() {
     _subscription = null;
     _futureOrPrefetch = null;
     _current = null;
     _state = _STATE_DONE;
   }
 
@@ skipping 44 matching lines @@
       _Future<bool> hasNext = _futureOrPrefetch;
       _clear();
       hasNext._complete(false);
       return;
     }
     _subscription.pause();
     _futureOrPrefetch = null;
     _state = _STATE_EXTRA_DONE;
   }
 }