Move remaining helpers to BackendHelpers

pkg/compiler/lib/src/js_backend/checked_mode_helpers.dart

 // Copyright (c) 2013, 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.
 
 part of js_backend;
 
 class CheckedModeHelper {
   final String name;
 
   const CheckedModeHelper(String this.name);
 
   Element getElement(Compiler compiler) {
     JavaScriptBackend backend = compiler.backend;
-    return backend.findHelper(name);
+    return backend.helpers.findHelper(name);
   }
 
   jsAst.Expression generateCall(SsaCodeGenerator codegen,
                                 HTypeConversion node) {
     Element helperElement = getElement(codegen.compiler);
     codegen.registry.registerStaticUse(helperElement);
     List<jsAst.Expression> arguments = <jsAst.Expression>[];
     codegen.use(node.checkedInput);
     arguments.add(codegen.pop());
     generateAdditionalArguments(codegen, node, arguments);
@@ skipping 89 matching lines @@
     DartType type = node.typeExpression;
     Element element = type.element;
     jsAst.Name isField = codegen.backend.namer.operatorIs(element);
     arguments.add(js.quoteName(isField));
     codegen.use(node.typeRepresentation);
     arguments.add(codegen.pop());
     jsAst.Name asField = codegen.backend.namer.substitutionName(element);
     arguments.add(js.quoteName(asField));
   }
 }