cps-ir: Don't bail on assert in unchecked mode.

tests/compiler/dart2js/js_backend_cps_ir_basic_test.dart

 // Copyright (c) 2014, 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.
 // VMOptions=-DUSE_CPS_IR=true
 
 // Tests for basic functionality.
 
 library basic_tests;
 
 import 'js_backend_cps_ir.dart';
@@ skipping 152 matching lines @@
   P.print(v0);
   return null;
 }"""),
   const TestEntry("""
 set foo(x) { print(x); }
 main() { foo = 42; }
 """, r"""
 function() {
   V.foo(42);
   return null;
+}"""),
+  // Assert
+  const TestEntry("""
+foo() { print('X'); }
+main() {
+  assert(true);
+  assert(false);
+  assert(foo());
+  print('Done');
+}""", r"""
+function() {
+  P.print("Done");
+  return null;
 }""")
 ];
 
 
 void main() {
   runTests(tests);
 }