Extract BackendUsage, MirrorsData, and CheckedModeHelpers from Backend.

pkg/compiler/lib/src/ssa/builder_kernel.dart

 // Copyright (c) 2016, 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.
 
 import 'package:kernel/ast.dart' as ir;
 
 import '../closure.dart';
 import '../common.dart';
 import '../common/codegen.dart' show CodegenRegistry, CodegenWorkItem;
 import '../common/names.dart';
@@ skipping 2401 matching lines @@
     if (_unexpectedForeignArguments(invocation, 1, 1)) {
       stack.add(
           // Result expected on stack.
           graph.addConstantBool(false, closedWorld));
       return;
     }
     String name = _foreignConstantStringArgument(invocation, 0, 'JS_GET_FLAG');
     bool value = false;
     switch (name) {
       case 'MUST_RETAIN_METADATA':
-        value = backend.mustRetainMetadata;
+        value = backend.mirrorsData.mustRetainMetadata;
         break;
       case 'USE_CONTENT_SECURITY_POLICY':
         value = compiler.options.useContentSecurityPolicy;
         break;
       default:
         compiler.reporter.reportErrorMessage(
             astAdapter.getNode(invocation),
             MessageKind.GENERIC,
             {'text': 'Error: Unknown internal flag "$name".'});
     }
@@ skipping 794 matching lines @@
     enterBlock.setBlockFlow(
         new HTryBlockInformation(
             kernelBuilder.wrapStatementGraph(bodyGraph),
             exception,
             kernelBuilder.wrapStatementGraph(catchGraph),
             kernelBuilder.wrapStatementGraph(finallyGraph)),
         exitBlock);
     kernelBuilder.inTryStatement = previouslyInTryStatement;
   }
 }