Add a "librarySource" field to CompilationUnitElementImpl.

pkg/analyzer/lib/src/task/incremental_element_builder.dart

Index: pkg/analyzer/lib/src/task/incremental_element_builder.dart
diff --git a/pkg/analyzer/lib/src/task/incremental_element_builder.dart b/pkg/analyzer/lib/src/task/incremental_element_builder.dart
index 7058c8630efc51e381522feb41a64d3af177418d..a01122dae50b314def9fa90d85546fffcd756be4 100644
--- a/pkg/analyzer/lib/src/task/incremental_element_builder.dart
+++ b/pkg/analyzer/lib/src/task/incremental_element_builder.dart
@@ -18,6 +18,7 @@ import 'package:analyzer/src/generated/source.dart';
  */
 class IncrementalCompilationUnitElementBuilder {
   final Source source;
+  final Source librarySource;
   final CompilationUnit oldUnit;
   final CompilationUnitElementImpl unitElement;
   final CompilationUnit newUnit;
@@ -27,10 +28,12 @@ class IncrementalCompilationUnitElementBuilder {
       CompilationUnit oldUnit, this.newUnit)
       : oldUnit = oldUnit,
         unitElement = oldUnit.element,
-        source = oldUnit.element.source;
+        source = oldUnit.element.source,
+        librarySource = (oldUnit.element as CompilationUnitElementImpl).librarySource;
 
   void build() {
-    new CompilationUnitBuilder().buildCompilationUnit(source, newUnit);
+    new CompilationUnitBuilder().buildCompilationUnit(
+        source, newUnit, librarySource);
     _processDirectives();
     _processUnitMembers();
     newUnit.element = unitElement;