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

pkg/compiler/lib/src/js_model/js_strategy.dart

Index: pkg/compiler/lib/src/js_model/js_strategy.dart
diff --git a/pkg/compiler/lib/src/js_model/js_strategy.dart b/pkg/compiler/lib/src/js_model/js_strategy.dart
index 6ac16ac3a025236f61fdccaeca19ec543f388267..a1aaef41b053c77dc5169f5c6737880660d1fe38 100644
--- a/pkg/compiler/lib/src/js_model/js_strategy.dart
+++ b/pkg/compiler/lib/src/js_model/js_strategy.dart
@@ -5,6 +5,7 @@
 library dart2js.js_model.strategy;
 
 import '../closure.dart' show ClosureConversionTask;
+import '../common.dart';
 import '../common/tasks.dart';
 import '../common_elements.dart';
 import '../compiler.dart';
@@ -208,6 +209,11 @@ class JsBackendStrategy implements KernelBackendStrategy {
         closedWorld,
         selectorConstraintsStrategy);
   }
+
+  @override
+  SourceSpan spanFromSpannable(Spannable spannable, Entity currentElement) {
+    return _elementMap.getSourceSpan(spannable, currentElement);
+  }
 }
 
 class JsRuntimeTypesNeed implements RuntimeTypesNeed {