Extract BackendUsageBuilder from BackendUsage

pkg/compiler/lib/src/deferred_load.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 deferred_load;
 
 import 'common/tasks.dart' show CompilerTask;
 import 'common.dart';
 import 'compiler.dart' show Compiler;
 import 'constants/expressions.dart' show ConstantExpression;
@@ skipping 13 matching lines @@
         Elements,
         ExportElement,
         FunctionElement,
         ImportElement,
         LibraryElement,
         MetadataAnnotation,
         PrefixElement,
         ResolvedAstKind,
         TypedefElement;
 import 'js_backend/js_backend.dart' show JavaScriptBackend;
+import 'js_backend/backend_usage.dart' show BackendUsage;
 import 'resolution/resolution.dart' show AnalyzableElementX;
 import 'resolution/tree_elements.dart' show TreeElements;
 import 'tree/tree.dart' as ast;
 import 'universe/use.dart' show StaticUse, StaticUseKind, TypeUse, TypeUseKind;
 import 'universe/world_impact.dart'
     show ImpactUseCase, WorldImpact, WorldImpactVisitorImpl;
 import 'util/setlet.dart' show Setlet;
 import 'util/uri_extras.dart' as uri_extras;
 import 'util/util.dart' show makeUnique;
 
@@ skipping 107 matching lines @@
 
   final Compiler compiler;
   DeferredLoadTask(Compiler compiler)
       : compiler = compiler,
         super(compiler.measurer) {
     mainOutputUnit.imports.add(_fakeMainImport);
   }
 
   JavaScriptBackend get backend => compiler.backend;
   DiagnosticReporter get reporter => compiler.reporter;
+  BackendUsage get _backendUsage => backend.backendUsage;
 
   /// Returns the [OutputUnit] where [element] belongs.
   OutputUnit outputUnitForElement(Element element) {
     if (!isProgramSplit) return mainOutputUnit;
 
     element = element.implementation;
     while (!_elementToOutputUnit.containsKey(element)) {
       // TODO(21051): workaround: it looks like we output annotation constants
       // for classes that we don't include in the output. This seems to happen
       // when we have reflection but can see that some classes are not needed.
@@ skipping 502 matching lines @@
         () => measure(() {
               // Starting from main, traverse the program and find all
               // dependencies.
               _mapDependencies(
                   element: compiler.mainFunction, import: _fakeMainImport);
 
               // Also add "global" dependencies to the main OutputUnit.  These
               // are things that the backend needs but cannot associate with a
               // particular element, for example, startRootIsolate.  This set
               // also contains elements for which we lack precise information.
-              for (Element element in backend.backendUsage.globalDependencies) {
+              for (Element element in _backendUsage.globalDependencies) {
                 _mapDependencies(element: element, import: _fakeMainImport);
               }
 
               // Now check to see if we have to add more elements due to
               // mirrors.
               if (compiler.commonElements.mirrorsLibrary != null) {
                 _addMirrorElements();
               }
 
               // Build the OutputUnits using these two maps.
@@ skipping 356 matching lines @@
 
   bool operator ==(other) {
     if (other is! _DeclaredDeferredImport) return false;
     return declaration == other.declaration;
   }
 
   int get hashCode => declaration.hashCode * 17;
 
   String toString() => '$declaration';
 }