Add support for external summaries bundle in AnalysisDriver.

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

Index: pkg/analyzer/lib/src/dart/analysis/library_analyzer.dart
diff --git a/pkg/analyzer/lib/src/dart/analysis/library_analyzer.dart b/pkg/analyzer/lib/src/dart/analysis/library_analyzer.dart
index acbf42300b4ae72a7b4fc96d5936ad918966cccb..4987624e24c485b77471c59be6a9b6a5434db4c8 100644
--- a/pkg/analyzer/lib/src/dart/analysis/library_analyzer.dart
+++ b/pkg/analyzer/lib/src/dart/analysis/library_analyzer.dart
@@ -758,6 +758,12 @@ class _ContentCacheWrapper implements ContentCache {
 
   @override
   bool getExists(Source source) {
+    if (fsState.externalSummaries != null) {
+      String uriStr = source.uri.toString();
+      if (fsState.externalSummaries.hasUnlinkedUnit(uriStr)) {
+        return true;
+      }
+    }
     return _getFileForSource(source).exists;
   }