Extract BackendUsage, MirrorsData, and CheckedModeHelpers from Backend.

pkg/compiler/lib/src/js_emitter/full_emitter/emitter.dart

 // Copyright (c) 2014, 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 dart2js.js_emitter.full_emitter;
 
 import 'dart:collection' show HashMap;
 import 'dart:convert';
 
 import 'package:js_runtime/shared/embedded_names.dart' as embeddedNames;
@@ skipping 429 matching lines @@
   /// The reflection name of a method 'foo' is 'foo:N:M:O', where N is the
   /// number of required arguments, M is the number of optional arguments, and
   /// O is the named arguments.
   /// The reflection name of a constructor is similar to a regular method but
   /// starts with 'new '.
   /// The reflection name of class 'C' is 'C'.
   /// An anonymous mixin application has no reflection name.
   /// This is used by js_mirrors.dart.
   String getReflectionName(elementOrSelector, jsAst.Name mangledName) {
     String name = elementOrSelector.name;
-    if (backend.shouldRetainName(name) ||
+    if (backend.mirrorsData.shouldRetainName(name) ||
         elementOrSelector is Element &&
             // Make sure to retain names of unnamed constructors, and
             // for common native types.
             ((name == '' &&
-                    backend.isAccessibleByReflection(elementOrSelector)) ||
+                    backend.mirrorsData
+                        .isAccessibleByReflection(elementOrSelector)) ||
                 _isNativeTypeNeedingReflectionName(elementOrSelector))) {
       // TODO(ahe): Enable the next line when I can tell the difference between
       // an instance method and a global.  They may have the same mangled name.
       // if (recordedMangledNames.contains(mangledName)) return null;
       recordedMangledNames.add(mangledName);
       return getReflectionNameInternal(elementOrSelector, mangledName);
     }
     return null;
   }
 
@@ skipping 566 matching lines @@
       LibraryElement library, Fragment fragment) {
     var uri = "";
     if (!compiler.options.enableMinification || backend.mustPreserveUris) {
       uri = library.canonicalUri;
       if (uri.scheme == 'file' && compiler.options.outputUri != null) {
         uri =
             relativize(compiler.options.outputUri, library.canonicalUri, false);
       }
     }
 
-    String libraryName =
-        (!compiler.options.enableMinification || backend.mustRetainLibraryNames)
-            ? library.libraryName
-            : "";
+    String libraryName = (!compiler.options.enableMinification ||
+            backend.mirrorsData.mustRetainLibraryNames)
+        ? library.libraryName
+        : "";
 
     jsAst.Fun metadata = task.metadataCollector.buildMetadataFunction(library);
 
     ClassBuilder descriptor = elementDescriptors[fragment][library];
 
     jsAst.ObjectInitializer initializer;
     if (descriptor == null) {
       // Nothing of the library was emitted.
       // TODO(floitsch): this should not happen. We currently have an example
       // with language/prefix6_negative_test.dart where we have an instance
@@ skipping 880 matching lines @@
     // data.
     mapping["_comment"] = "This mapping shows which compiled `.js` files are "
         "needed for a given deferred library import.";
     mapping.addAll(compiler.deferredLoadTask.computeDeferredMap());
     compiler.outputProvider(
         compiler.options.deferredMapUri.path, 'deferred_map')
       ..add(const JsonEncoder.withIndent("  ").convert(mapping))
       ..close();
   }
 }