Fix issue 2939, revert previous change that accepted null as excpetion objects:

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 457 matching lines @@
 type_variable_bounds2_test/02: Pass # For the wrong reasons.
 type_variable_bounds2_test/03: Fail
 type_variable_bounds2_test/05: Fail
 type_variable_bounds2_test/06: Pass # For the wrong reasons.
 assign_top_method_negative_test: Pass # For the wrong reasons.
 
 
 [ $compiler == dart2js]
 compile_time_constant8_test: Fail # We don't take the generic type into account yet.
 static_initializer_type_error_test: Fail # Checked mode not supported.
-
+exception_test: Fail # "throw null" throws NullPointerException.
 
 # Compilation errors.
 ct_const_test: Fail # We don't take the generic type into account yet.
 call_operator_test: Fail # Unexpected token 'call'
 char_escape_test: Fail # Unhandled non-BMP character: U+10000
 constructor6_test: Fail # Closures inside initializers not implemented.
 default_factory_library_test: Fail # lib is not a type
 dynamic_test: Fail # cannot resolve type F1
 factory3_test: Fail # internal error: visitIs for type variables not implemented
 function_literals2_test: Fail # Closures inside initializers not implemented.
@@ skipping 180 matching lines @@
 
 list_test: Fail, OK # Obsolete test uses List.copyFrom that was removed on 2011-10-31.
 
 final_syntax_test/none: Fail, OK # Uses deprecated "StrConstant + StrConstant".
 
 [ $compiler == dart2js && $mode == release ]
 assign_top_method_negative_test: Crash
 
 [ $compiler == dart2js && $runtime == none ]
 *: Fail, Pass # TODO(ahe): Triage these tests.