Fix warnings and hints in the SDK

sdk/lib/async/stream_impl.dart

Index: sdk/lib/async/stream_impl.dart
diff --git a/sdk/lib/async/stream_impl.dart b/sdk/lib/async/stream_impl.dart
index 3f77294619a6e3d3b1e23f7794fc6b51e8700e95..df03b9ba08498ccadf7955637efc625adff67d56 100644
--- a/sdk/lib/async/stream_impl.dart
+++ b/sdk/lib/async/stream_impl.dart
@@ -1017,6 +1017,8 @@ class _StreamIteratorImpl<T> implements StreamIterator<T> {
           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. */