dart2js cps: Emit 'return' after throwing call to assist VM analysis.

tests/compiler/dart2js/js_backend_cps_ir_gvn_test.dart

 // Copyright (c) 2015, 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.
 
 // Test that the GVN optimization pass works as expected.
 
 library basic_tests;
 
 import 'js_backend_cps_ir.dart';
 
@@ skipping 36 matching lines @@
   var y1 = new Branch(x1, x2);
   var y2 = new Branch(x3, x4);
   var z  = new Root(y1, y2);
   print(foo(z, [1,2,3,4,5,6,7,8,9,10]));
 }
 """,r"""
 function(x, list) {
   var v0 = x.left, a = v0.left, b = v0.right, sum = 0, k = 0, c = (v0 = x.right).left, d = v0.right, i = a.value + c.value, v1 = list[v0 = i * (b.value + d.value)];
   for (; k < 10; sum = sum + (v1 + i), k = k + 1)
     if (v0 < 0 || v0 >= 10)
-      H.ioore(list, v0);
+      return H.ioore(list, v0);
   return sum;
 }"""),
 ];
 
 void main() {
   runTests(tests);
 }