dart2js: Use an abstract Name class for names in the generated JavaScript ast.

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);
@@ skipping 70 matching lines @@
   }
 }
 
 class PropertyCheckedModeHelper extends CheckedModeHelper {
   const PropertyCheckedModeHelper(String name) : super(name);
 
   void generateAdditionalArguments(SsaCodeGenerator codegen,
                                    HTypeConversion node,
                                    List<jsAst.Expression> arguments) {
     DartType type = node.typeExpression;
-    String additionalArgument = codegen.backend.namer.operatorIsType(type);
-    arguments.add(js.string(additionalArgument));
+    jsAst.Name additionalArgument = codegen.backend.namer.operatorIsType(type);
+    arguments.add(js.quoteName(additionalArgument));
   }
 }
 
 class TypeVariableCheckedModeHelper extends CheckedModeHelper {
   const TypeVariableCheckedModeHelper(String name) : super(name);
 
   void generateAdditionalArguments(SsaCodeGenerator codegen,
                                    HTypeConversion node,
                                    List<jsAst.Expression> arguments) {
     assert(node.typeExpression.isTypeVariable);
     codegen.use(node.typeRepresentation);
     arguments.add(codegen.pop());
   }
 }
 
 class SubtypeCheckedModeHelper extends CheckedModeHelper {
   const SubtypeCheckedModeHelper(String name) : super(name);
 
   void generateAdditionalArguments(SsaCodeGenerator codegen,
                                    HTypeConversion node,
                                    List<jsAst.Expression> arguments) {
     DartType type = node.typeExpression;
     Element element = type.element;
-    String isField = codegen.backend.namer.operatorIs(element);
-    arguments.add(js.string(isField));
+    jsAst.Name isField = codegen.backend.namer.operatorIs(element);
+    arguments.add(js.quoteName(isField));
     codegen.use(node.typeRepresentation);
     arguments.add(codegen.pop());
-    String asField = codegen.backend.namer.substitutionName(element);
-    arguments.add(js.string(asField));
+    jsAst.Name asField = codegen.backend.namer.substitutionName(element);
+    arguments.add(js.quoteName(asField));
   }
 }