set LibraryElement documentation to docs of first directive in CU

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

Index: pkg/analyzer/lib/src/task/dart.dart
diff --git a/pkg/analyzer/lib/src/task/dart.dart b/pkg/analyzer/lib/src/task/dart.dart
index 56fc384bb758da3271b428e29ba36ea35e3b5642..482ece8b5d29a490e0ea552d5d709dbcf1865c7a 100644
--- a/pkg/analyzer/lib/src/task/dart.dart
+++ b/pkg/analyzer/lib/src/task/dart.dart
@@ -1492,9 +1492,12 @@ class BuildLibraryElementTask extends SourceBasedAnalysisTask {
       directive.element = libraryElement;
     }
     BuildLibraryElementUtils.patchTopLevelAccessors(libraryElement);
-    if (libraryDirective != null) {
-      _setDoc(libraryElement, libraryDirective);
+    // set the library documentation to the docs associated with the first
+    // directive in the compilation unit.
+    if (definingCompilationUnit.directives.isNotEmpty) {
+      _setDoc(libraryElement, definingCompilationUnit.directives.first);

[Brian Wilkerson, 2016/01/22 22:44:52]

Given that this was the only use of `libraryDirective`, it would be good to
remove it.

[keertip, 2016/01/23 00:12:04]

Done.

     }
+
     //
     // Record outputs.
     //