Disable the irregexp backend.

runtime/lib/regexp.cc

 // 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.
 
 #include "platform/assert.h"
 #include "vm/bootstrap_natives.h"
 #include "vm/exceptions.h"
 #include "vm/native_entry.h"
 #include "vm/object.h"
 #include "vm/regexp_parser.h"
 
 #include "lib/regexp_jsc.h"
 
 namespace dart {
 
 DECLARE_FLAG(bool, trace_irregexp);
-DEFINE_FLAG(bool, use_jscre, false, "Use the JSCRE regular expression engine");
+DEFINE_FLAG(bool, use_jscre, true, "Use the JSCRE regular expression engine");
 
 
 DEFINE_NATIVE_ENTRY(JSSyntaxRegExp_factory, 4) {
   ASSERT(TypeArguments::CheckedHandle(arguments->NativeArgAt(0)).IsNull());
   GET_NON_NULL_NATIVE_ARGUMENT(String, pattern, arguments->NativeArgAt(1));
   GET_NON_NULL_NATIVE_ARGUMENT(
       Instance, handle_multi_line, arguments->NativeArgAt(2));
   GET_NON_NULL_NATIVE_ARGUMENT(
       Instance, handle_case_sensitive, arguments->NativeArgAt(3));
   bool ignore_case = handle_case_sensitive.raw() != Bool::True().raw();
@@ skipping 71 matching lines @@
 
   // Retrieve the cached function.
   const Function& fn = Function::Handle(regexp.function(cid));
   ASSERT(!fn.IsNull());
 
   // And finally call the generated code.
   return IRRegExpMacroAssembler::Execute(fn, str, start_index, isolate);
 }
 
 }  // namespace dart