Fix race condition with 'knownFiles'.

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

Index: 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 54c526ec1f63a8f1b03779068ea39d16d236ec4a..d8e7ba244fab2309b9fbbab1a28cf053da77cb1f 100644
--- a/pkg/analyzer/lib/src/dart/analysis/driver.dart
+++ b/pkg/analyzer/lib/src/dart/analysis/driver.dart
@@ -278,7 +278,13 @@ class AnalysisDriver {
   /**
    * Return the set of files that are known, i.e. added or used implicitly.
    */
-  Set<String> get knownFiles => _fsState.knownFilePaths;
+  Set<String> get knownFiles {
+    // TODO(scheglov) This method is invoked too often and the implementation
+    // is probably not the most efficient. Consider isKnownFile().
+    return new Set<String>()
+      ..addAll(_explicitFiles)
+      ..addAll(_fsState.knownFilePaths);
+  }
 
   /**
    * Return the list of files that the driver should try to analyze sooner.