Remove saving/restoring of the context at function entry.

This is not needed anymore after I changed the current context
to always reside in a local variable.

Further simplifications and cleanup in the debugger.

This also fixes a bad memory retention problem with
non-capturing closures.

BUG=dartbug.com/18886
TEST=tests/language/vm/closure_memory_retention_test.dart
R=hausner@google.com

Review URL: https://codereview.chromium.org//695483003

git-svn-id: https://dart.googlecode.com/svn/branches/bleeding_edge@41433 260f80e4-7a28-3924-810f-c04153c831b5