Fix source information positions for deserialized patched elements.

pkg/compiler/lib/src/cps_ir/cps_ir_builder_task.dart

Index: pkg/compiler/lib/src/cps_ir/cps_ir_builder_task.dart
diff --git a/pkg/compiler/lib/src/cps_ir/cps_ir_builder_task.dart b/pkg/compiler/lib/src/cps_ir/cps_ir_builder_task.dart
index e7595b4edf6ce7356be00eba242a431ba46ed1e9..c382e4e5f5947a316889bf6fe1d1a12e7608acd0 100644
--- a/pkg/compiler/lib/src/cps_ir/cps_ir_builder_task.dart
+++ b/pkg/compiler/lib/src/cps_ir/cps_ir_builder_task.dart
@@ -78,7 +78,7 @@ class IrBuilderTask extends CompilerTask {
       element = element.implementation;
       return reporter.withCurrentElement(element, () {
         SourceInformationBuilder sourceInformationBuilder =
-            sourceInformationStrategy.createBuilderForContext(element);
+            sourceInformationStrategy.createBuilderForContext(resolvedAst);
 
         IrBuilderVisitor builder = new IrBuilderVisitor(
             resolvedAst, compiler, sourceInformationBuilder, typeMaskSystem);
@@ -502,8 +502,9 @@ class IrBuilderVisitor extends ast.Visitor<ir.Primitive>
   /// Every visitor can only be applied to nodes in one context, because
   /// the [elements] field is specific to that context.
   IrBuilderVisitor makeVisitorForContext(AstElement context) {
-    return new IrBuilderVisitor(context.resolvedAst, compiler,
-        sourceInformationBuilder.forContext(context), typeMaskSystem);
+    ResolvedAst resolvedAst = backend.frontend.getResolvedAst(context);
+    return new IrBuilderVisitor(resolvedAst, compiler,
+        sourceInformationBuilder.forContext(resolvedAst), typeMaskSystem);
   }
 
   /// Builds the IR for an [expression] taken from a different [context].