Add support for rethrow and start treating throw <expr> as an expression.

tests/language/language.status

 # 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.
 
 # This directory contains tests that are intended to show the
 # current state of the language.
 
 # In order to maintain maximum test coverage for all builds,
 # please use the following procedure to mark a test
 # failed on architectures other than the one you are working on.
@@ skipping 407 matching lines @@
 final_syntax_test/02: Fail # http://dartbug.com/5519
 final_syntax_test/03: Fail # http://dartbug.com/5519
 final_syntax_test/04: Fail # http://dartbug.com/5519
 getter_no_setter_test/01: Fail # http://dartbug.com/5519
 isnot_malformed_type_test/01: Fail # http://dartbug.com/5519
 method_override2_test/01: Fail # http://dartbug.com/5519
 named_parameters_aggregated_test/01: Fail # http://dartbug.com/5519
 named_parameters_aggregated_test/03: Fail # http://dartbug.com/5519
 not_enough_positional_arguments_test/01: Fail # http://dartbug.com/5519
 
-throw_expr_test: Fail
 metadata_test: Fail
 # Fails in conservative mode, issue 4935, passes in minifinying mode.
 bad_constructor_test/04: Fail
 bad_constructor_test/05: Fail
 bad_constructor_test/06: Fail
 argument_definition_test/*: Skip # Not implemented.
 argument_definition2_test: Skip # Not implemented. Fails in minified tests.
 argument_definition3_test: Skip # Not implemented. Fails in minified tests.
 argument_definition4_test: Skip # Not implemented. Fails in minified tests.
 const_var_test: Pass, Fail # Map literals take 2 type arguments.
@@ skipping 109 matching lines @@
 type_variable_field_initializer_closure_test: Crash # VM bug: issue 8847
 
 super_getter_setter_test: Fail # VM bug: issue 8917
 super_operator_index7_test: Fail # VM bug: issue 8918
 
 execute_finally10_test: Fail # VM bug: issue 430
 execute_finally11_test: Fail # VM bug: issue 430
 
 [ $compiler == dart2dart && $minified ]
 
-rethrow_test: Fail
-
 # TODO(tball): Assign proper bug numbers.
 class_literal_test/none: Fail
 
 import_core_prefix_test: Pass
 prefix22_test: Pass
 invocation_mirror_test: Fail, OK # hardcoded names.
 super_call4_test: Fail, OK # hardcoded names.
 
 [ $arch == arm ]
 *: Skip
 
 [ $arch == simarm ]
 *: Skip
 
 [ $arch == mips ]
 *: Skip
 
 [ $arch == simmips ]
 *: Skip