Use JavaScript runtime semantics when constant folding.

lib/compiler/implementation/js_backend/backend.dart

 // 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.
 
 typedef void Recompile(Element element);
 
 class ReturnInfo {
   HType returnType;
   List<Element> compiledFunctions;
 
@@ skipping 303 matching lines @@
     return <CompilerTask>[builder, optimizer, generator, emitter];
   }
 
   JavaScriptBackend(Compiler compiler, bool generateSourceMap)
       : emitter = new CodeEmitterTask(compiler, generateSourceMap),
         fieldInitializers = new Map<Element, Map<Element, HType>>(),
         fieldConstructorSetters = new Map<Element, Map<Element, HType>>(),
         fieldSettersType = new Map<Element, Map<Element, HType>>(),
         returnInfo = new Map<Element, ReturnInfo>(),
         invalidateAfterCodegen = new List<Element>(),
-        super(compiler) {
+        super(compiler, constantSystem: JAVA_SCRIPT_CONSTANT_SYSTEM) {
     builder = new SsaBuilderTask(this);
     optimizer = new SsaOptimizerTask(this);
     generator = new SsaCodeGeneratorTask(this);
     argumentTypes = new ArgumentTypesRegistry(this);
   }
 
   Element get cyclicThrowHelper() {
     return compiler.findHelper(const SourceString("throwCyclicInit"));
   }
 
@@ skipping 259 matching lines @@
           return const SourceString('listSuperTypeCheck');
         }
       } else if (nativeCheck) {
         return const SourceString('callTypeCheck');
       } else {
         return const SourceString('propertyTypeCheck');
       }
     }
   }
 }