Compute source spans for IR nodes and J/K-elements

pkg/compiler/lib/src/kernel/kernel_strategy.dart

 // Copyright (c) 2017, 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.kernel.frontend_strategy;
 
 import '../closure.dart';
 import '../common.dart';
 import '../common_elements.dart';
 import '../common/backend_api.dart';
@@ skipping 125 matching lines @@
   WorkItemBuilder createResolutionWorkItemBuilder(
       NativeBasicData nativeBasicData,
       NativeDataBuilder nativeDataBuilder,
       ImpactTransformer impactTransformer) {
     return new KernelWorkItemBuilder(
         elementMap, nativeBasicData, nativeDataBuilder, impactTransformer);
   }
 
   @override
   SourceSpan spanFromSpannable(Spannable spannable, Entity currentElement) {
-    // TODO(redemption): Compute source spans from kernel elements.
-    return new SourceSpan(null, null, null);
+    return _elementMap.getSourceSpan(spannable, currentElement);
   }
 }
 
 class KernelWorkItemBuilder implements WorkItemBuilder {
   final KernelToElementMapForImpactImpl _elementMap;
   final ImpactTransformer _impactTransformer;
   final NativeMemberResolver _nativeMemberResolver;
 
   KernelWorkItemBuilder(this._elementMap, NativeBasicData nativeBasicData,
       NativeDataBuilder nativeDataBuilder, this._impactTransformer)
@@ skipping 73 matching lines @@
   int get preMirrorsMethodCount {
     // TODO(redemption): Implement this.
     return null;
   }
 
   @override
   void onQueueEmpty(Enqueuer enqueuer, Iterable<ClassEntity> recentClasses) {
     // TODO(redemption): Implement this.
   }
 }