Fix recomputing LibraryElement.exportNamespace.

Fix recomputing LibraryElement.exportNamespace.

This caused the bogus warnings in Flutter.

1. object.dart exports painting.dart
2. painting.dart exports box_painter.dart
3. When the user changes box_painter.dart, we correctly invalidate
   LIBRARY_ELEMENT4, which contains LibraryElement.exportNamespace.
4. When BuildExportNamespaceTask recomputes LIBRARY_ELEMENT4, and
   recomputes exportNamespace for the library... it calls
   NamespaceBuilder.createExportNamespaceForLibrary(), which uses the
   cached LibraryElement.exportNamespace, which we don't flush.


R=brianwilkerson@google.com, pquitslund@google.com
BUG=

Committed: https://github.com/dart-lang/sdk/commit/662a461988b5097a0e7f48628e4330aaf9447b59

[Brian Wilkerson, 2016-06-23 13:14:54]

lgtm

[scheglov, 2016-06-23 15:39:46]

Description was changed from

==========
Fix recomputing LibraryElement.exportNamespace.

This caused the bogus warnings in Flutter.

1. object.dart exports painting.dart
2. painting.dart exports box_painter.dart
3. When the user changes box_painter.dart, we correctly invalidate
   LIBRARY_ELEMENT4, which contains LibraryElement.exportNamespace.
4. When BuildExportNamespaceTask recomputes LIBRARY_ELEMENT4, and
   recomputes exportNamespace for the library... it calls
   NamespaceBuilder.createExportNamespaceForLibrary(), which uses the
   cached LibraryElement.exportNamespace, which we don't flush.


R=brianwilkerson@google.com,pquitslund@google.com
BUG=
==========

to

==========
Fix recomputing LibraryElement.exportNamespace.

This caused the bogus warnings in Flutter.

1. object.dart exports painting.dart
2. painting.dart exports box_painter.dart
3. When the user changes box_painter.dart, we correctly invalidate
   LIBRARY_ELEMENT4, which contains LibraryElement.exportNamespace.
4. When BuildExportNamespaceTask recomputes LIBRARY_ELEMENT4, and
   recomputes exportNamespace for the library... it calls
   NamespaceBuilder.createExportNamespaceForLibrary(), which uses the
   cached LibraryElement.exportNamespace, which we don't flush.


R=brianwilkerson@google.com, pquitslund@google.com
BUG=

Committed:
https://github.com/dart-lang/sdk/commit/662a461988b5097a0e7f48628e4330aaf9447b59
==========

[scheglov, 2016-06-23 15:39:48]

Committed patchset #1 (id:1) manually as
662a461988b5097a0e7f48628e4330aaf9447b59 (presubmit successful).