Base JavaScript code position computation on JavaScript tracer.

pkg/compiler/lib/src/js_backend/backend.dart

Index: pkg/compiler/lib/src/js_backend/backend.dart
diff --git a/pkg/compiler/lib/src/js_backend/backend.dart b/pkg/compiler/lib/src/js_backend/backend.dart
index 5a467f5515caa27cf5ef87bf27bd1ff76d74db69..699ec1dbdee8a7faeb69a6af3b279bf948c21e80 100644
--- a/pkg/compiler/lib/src/js_backend/backend.dart
+++ b/pkg/compiler/lib/src/js_backend/backend.dart
@@ -502,7 +502,7 @@ class JavaScriptBackend extends Backend {
 
   bool enabledNoSuchMethod = false;
 
-  final SourceInformationStrategy sourceInformationStrategy;
+  SourceInformationStrategy sourceInformationStrategy;
 
   final BackendHelpers helpers;
   final BackendImpacts impacts;
@@ -520,7 +520,7 @@ class JavaScriptBackend extends Backend {
         this.sourceInformationStrategy =
             generateSourceMap
                 ? (useNewSourceInfo
-                     ? const PositionSourceInformationStrategy()
+                     ? new PositionSourceInformationStrategy()
                      : const StartEndSourceInformationStrategy())
                 : const JavaScriptSourceInformationStrategy(),
         helpers = new BackendHelpers(compiler),
@@ -1630,7 +1630,12 @@ class JavaScriptBackend extends Backend {
       }
     }
 
-    generatedCode[element] = functionCompiler.compile(work);
+    jsAst.Fun function = functionCompiler.compile(work);
+    if (function.sourceInformation == null) {
+      function = function.withSourceInformation(
+          sourceInformationStrategy.buildPreambleMarker());
+    }
+    generatedCode[element] = function;
     WorldImpact worldImpact =
         impactTransformer.transformCodegenImpact(work.registry.worldImpact);
     compiler.dumpInfoTask.registerImpact(element, worldImpact);