Implement JsKernelToElementMap through KernelToElementMapBase

pkg/compiler/lib/src/js_emitter/program_builder/program_builder.dart

Index: pkg/compiler/lib/src/js_emitter/program_builder/program_builder.dart
diff --git a/pkg/compiler/lib/src/js_emitter/program_builder/program_builder.dart b/pkg/compiler/lib/src/js_emitter/program_builder/program_builder.dart
index 44f2f4e498da4c5cf72d334adaa8db80243c5872..a8e04792ad9068c8e88799fd94d7b7a1a38ecb65 100644
--- a/pkg/compiler/lib/src/js_emitter/program_builder/program_builder.dart
+++ b/pkg/compiler/lib/src/js_emitter/program_builder/program_builder.dart
@@ -101,6 +101,9 @@ class ProgramBuilder {
   final CodeEmitterTask _task;
   final ClosedWorld _closedWorld;
 
+  /// The [Sorter] used for ordering elements in the generated JavaScript.
+  final Sorter _sorter;
+
   /// Contains the collected information the program builder used to build
   /// the model.
   // The collector will be filled on the first call to `buildProgram`.
@@ -143,6 +146,7 @@ class ProgramBuilder {
       this._namer,
       this._task,
       this._closedWorld,
+      this._sorter,
       Set<ClassEntity> rtiNeededClasses,
       this._mainFunction,
       {bool isMockCompilation})
@@ -163,8 +167,8 @@ class ProgramBuilder {
             _closedWorld,
             rtiNeededClasses,
             _generatedCode,
-            _task.sorter),
-        this._registry = new Registry(_deferredLoadTask, _task.sorter);
+            _sorter),
+        this._registry = new Registry(_deferredLoadTask, _sorter);
 
   /// Mapping from [ClassEntity] to constructed [Class]. We need this to
   /// update the superclass in the [Class].
@@ -195,8 +199,6 @@ class ProgramBuilder {
   /// interceptors, ...).
   Set<ClassElement> _notSoftDeferred;
 
-  Sorter get _sorter => _task.sorter;
-
   Program buildProgram({bool storeFunctionTypesInMetadata: false}) {
     collector.collect();
     _initializeSoftDeferredMap();