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

tests/language/language.status

Index: tests/language/language.status
diff --git a/tests/language/language.status b/tests/language/language.status
index f732024350b27337146d927ef79bcce42e96c63d..5897ea0ec02d5ab85e5f7c20d37c4154ef0e946e 100644
--- a/tests/language/language.status
+++ b/tests/language/language.status
@@ -425,7 +425,6 @@ 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
@@ -555,8 +554,6 @@ 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