Migrate DDC to the new analysis driver.

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 71f1f2093d0322979dfa9a94e84f8052e05aeab0..864e24c69ae695163a7f5e786ed4b7340aad93da 100644
--- a/pkg/analyzer/lib/src/dart/analysis/library_analyzer.dart
+++ b/pkg/analyzer/lib/src/dart/analysis/library_analyzer.dart
@@ -474,7 +474,8 @@ class LibraryAnalyzer {
       }
     }
 
-    if (hasPartDirective && libraryNameNode == null &&
+    if (hasPartDirective &&
+        libraryNameNode == null &&
         !_context.analysisOptions.enableUriInPartOf) {
       libraryErrorReporter.reportErrorForOffset(
           ResolverErrorCode.MISSING_LIBRARY_DIRECTIVE_WITH_PART, 0, 0);
@@ -740,6 +741,10 @@ class _ContentCacheWrapper implements ContentCache {
 
   @override
   bool getExists(Source source) {
+    if (fsState.externalSummaries != null &&
+        fsState.externalSummaries.hasUnlinkedUnit(source.uri.toString())) {
+      return true;
+    }
     return _getFileForSource(source).exists;
   }