Don't clear library context when nothing to do.

This is the first step, the next step is to keep some libraries
even when there is a change, but the changed file does not affect
these libraries.

Change-Id: I3b1388be7733f3725ca632b4a67909393d893915
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/206442
Reviewed-by: Brian Wilkerson <brianwilkerson@google.com>
Commit-Queue: Konstantin Shcheglov <scheglov@google.com>

pkg/analyzer/lib/src/dart/analysis/driver.dart

diff --git a/pkg/analyzer/lib/src/dart/analysis/driver.dart b/pkg/analyzer/lib/src/dart/analysis/driver.dart
index 79bc69f..f275425 100644
--- a/pkg/analyzer/lib/src/dart/analysis/driver.dart
+++ b/pkg/analyzer/lib/src/dart/analysis/driver.dart
@@ -463,7 +463,6 @@
         _unitElementRequestedParts.isNotEmpty) {
       return AnalysisDriverPriority.general;
     }
-    clearLibraryContext();
     return AnalysisDriverPriority.nothing;
   }