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

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

Index: pkg/compiler/lib/src/kernel/kernel_backend_strategy.dart
diff --git a/pkg/compiler/lib/src/kernel/kernel_backend_strategy.dart b/pkg/compiler/lib/src/kernel/kernel_backend_strategy.dart
index 1282ca16b61354c0d83eef1699f467ff40706600..bd1f97a391eadba432f63ee9200e2dc449c0bb99 100644
--- a/pkg/compiler/lib/src/kernel/kernel_backend_strategy.dart
+++ b/pkg/compiler/lib/src/kernel/kernel_backend_strategy.dart
@@ -8,6 +8,7 @@ import 'package:kernel/ast.dart' as ir;
 
 import '../backend_strategy.dart';
 import '../closure.dart';
+import '../common.dart';
 import '../common/codegen.dart' show CodegenRegistry, CodegenWorkItem;
 import '../common/tasks.dart';
 import '../compiler.dart';
@@ -119,6 +120,13 @@ class KernelBackendStrategyImpl implements KernelBackendStrategy {
   @override
   SourceInformationStrategy get sourceInformationStrategy =>
       const JavaScriptSourceInformationStrategy();
+
+  @override
+  SourceSpan spanFromSpannable(Spannable spannable, Entity currentElement) {
+    KernelFrontEndStrategy frontendStrategy = _compiler.frontendStrategy;
+    KernelToElementMapImpl elementMap = frontendStrategy.elementMap;
+    return elementMap.getSourceSpan(spannable, currentElement);
+  }
 }
 
 class KernelCodegenWorkItemBuilder implements WorkItemBuilder {