Add a "librarySource" field to CompilationUnitElementImpl.

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

Index: pkg/analyzer/lib/src/task/task_dart.dart
diff --git a/pkg/analyzer/lib/src/task/task_dart.dart b/pkg/analyzer/lib/src/task/task_dart.dart
index 5a3491b3c7c362375ef005e366029342312f5d48..919297051531f7e415178f5f4f6120123172e06b 100644
--- a/pkg/analyzer/lib/src/task/task_dart.dart
+++ b/pkg/analyzer/lib/src/task/task_dart.dart
@@ -82,6 +82,6 @@ class BuildUnitElementTask extends AnalysisTask {
   @override
   void internalPerform() {
     CompilationUnitBuilder builder = new CompilationUnitBuilder();
-    unitElement = builder.buildCompilationUnit(source, unit);
+    unitElement = builder.buildCompilationUnit(source, unit, library);
   }
 }