Prepare for late creation of ResolutionEnqueuer

pkg/compiler/lib/src/enqueue.dart

 // Copyright (c) 2012, 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.enqueue;
 
 import 'dart:collection' show Queue;
 
 import 'common/resolution.dart' show Resolution;
 import 'common/tasks.dart' show CompilerTask;
@@ skipping 17 matching lines @@
 
 class EnqueueTask extends CompilerTask {
   ResolutionEnqueuer _resolution;
   final Compiler compiler;
 
   String get name => 'Enqueue';
 
   EnqueueTask(Compiler compiler)
       : this.compiler = compiler,
         super(compiler.measurer) {
-    _resolution = new ResolutionEnqueuer(
-        this,
-        compiler.options,
-        compiler.reporter,
-        compiler.options.analyzeOnly && compiler.options.analyzeMain
-            ? const DirectEnqueuerStrategy()
-            : const TreeShakingEnqueuerStrategy(),
-        compiler.backend.resolutionEnqueuerListener,
-        new ElementResolutionWorldBuilder(
-            compiler.backend, compiler.resolution, const OpenWorldStrategy()),
-        new ResolutionWorkItemBuilder(compiler.resolution));
+    createResolutionEnqueuer();
   }
 
-  ResolutionEnqueuer get resolution => _resolution;
+  ResolutionEnqueuer get resolution {
+    assert(invariant(NO_LOCATION_SPANNABLE, _resolution != null,
+        message: "ResolutionEnqueuer has not been created yet."));
+    return _resolution;
+  }
+
+  ResolutionEnqueuer createResolutionEnqueuer() {
+    return _resolution ??=
+        compiler.backend.createResolutionEnqueuer(this, compiler);
+  }
 
   Enqueuer createCodegenEnqueuer(ClosedWorld closedWorld) {
     return compiler.backend.createCodegenEnqueuer(this, compiler, closedWorld);
   }
 }
 
 abstract class Enqueuer {
   WorldBuilder get worldBuilder;
 
   void open(ImpactStrategy impactStrategy, FunctionEntity mainMethod,
@@ skipping 499 matching lines @@
   @override
   WorkItem createWorkItem(MemberElement element) {
     assert(invariant(element, element.isDeclaration));
     if (element.isMalformed) return null;
 
     assert(invariant(element, element is AnalyzableElement,
         message: 'Element $element is not analyzable.'));
     return _resolution.createWorkItem(element);
   }
 }