Remove the checker and corresponding dead code

lib/src/codegen/js_codegen.dart

 // Copyright (c) 2015, 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.
 
 library dev_compiler.src.codegen.js_codegen;
 
 import 'dart:collection' show HashSet, HashMap, SplayTreeSet;
 
 import 'package:analyzer/analyzer.dart' hide ConstantEvaluator;
 import 'package:analyzer/src/generated/ast.dart' hide ConstantEvaluator;
 import 'package:analyzer/src/generated/constant.dart';
 import 'package:analyzer/src/generated/element.dart';
 import 'package:analyzer/src/generated/resolver.dart' show TypeProvider;
 import 'package:analyzer/src/generated/scanner.dart'
     show StringToken, Token, TokenType;
 import 'package:analyzer/src/task/dart.dart' show PublicNamespaceBuilder;
+import 'package:analyzer/src/task/strong/rules.dart';
 
 import 'ast_builder.dart' show AstBuilder;
 import 'reify_coercions.dart' show CoercionReifier, Tuple2;
 
 // TODO(jmesserly): import from its own package
 import '../js/js_ast.dart' as JS;
 import '../js/js_ast.dart' show js;
 
 import '../closure/closure_annotator.dart' show ClosureAnnotator;
 import '../compiler.dart' show AbstractCompiler;
-import '../checker/rules.dart';
 import '../info.dart';
 import '../options.dart' show CodegenOptions;
 import '../utils.dart';
 
 import 'code_generator.dart';
 import 'js_field_storage.dart';
 import 'js_names.dart' as JS;
 import 'js_metalet.dart' as JS;
 import 'js_module_item_order.dart';
 import 'js_printer.dart' show writeJsLibrary;
@@ skipping 3276 matching lines @@
 
 /// A special kind of element created by the compiler, signifying a temporary
 /// variable. These objects use instance equality, and should be shared
 /// everywhere in the tree where they are treated as the same variable.
 class TemporaryVariableElement extends LocalVariableElementImpl {
   TemporaryVariableElement.forNode(Identifier name) : super.forNode(name);
 
   int get hashCode => identityHashCode(this);
   bool operator ==(Object other) => identical(this, other);
 }