Manage known file paths set in FileSystemState.

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

Index: pkg/analyzer/lib/src/dart/analysis/file_state.dart
diff --git a/pkg/analyzer/lib/src/dart/analysis/file_state.dart b/pkg/analyzer/lib/src/dart/analysis/file_state.dart
index 7ab09c13f5d48125d2f2169a04b6ff344f68d020..ecb8b6750ef275fe40cfdac21dab185d14876168 100644
--- a/pkg/analyzer/lib/src/dart/analysis/file_state.dart
+++ b/pkg/analyzer/lib/src/dart/analysis/file_state.dart
@@ -548,6 +548,11 @@ class FileSystemState {
   final Map<Uri, FileState> _uriToFile = {};
 
   /**
+   * All known file paths.
+   */
+  final Set<String> knownFilePaths = new Set<String>();
+
+  /**
    * Mapping from a path to the corresponding [FileState]s, canonical or not.
    */
   final Map<String, List<FileState>> _pathToFiles = {};
@@ -577,11 +582,6 @@ class FileSystemState {
   }
 
   /**
-   * Return the set of known file paths.
-   */
-  Set<String> get knownFilePaths => _pathToFiles.keys.toSet();
-
-  /**
    * Return the known files.
    */
   Iterable<FileState> get knownFiles =>
@@ -614,7 +614,7 @@ class FileSystemState {
       FileSource uriSource = new FileSource(resource, uri);
       file = new FileState._(this, path, uri, uriSource);
       _uriToFile[uri] = file;
-      _pathToFiles.putIfAbsent(path, () => <FileState>[]).add(file);
+      _addFileWithPath(path, file);
       _pathToCanonicalFile[path] = file;
       file.refresh();
     }
@@ -640,7 +640,7 @@ class FileSystemState {
       FileSource source = new FileSource(resource, uri);
       file = new FileState._(this, path, uri, source);
       _uriToFile[uri] = file;
-      _pathToFiles.putIfAbsent(path, () => <FileState>[]).add(file);
+      _addFileWithPath(path, file);
       file.refresh();
     }
     return file;
@@ -660,6 +660,16 @@ class FileSystemState {
       ..remove(canonicalFile)
       ..insert(0, canonicalFile);
   }
+
+  void _addFileWithPath(String path, FileState file) {
+    var files = _pathToFiles[path];
+    if (files == null) {
+      knownFilePaths.add(path);
+      files = <FileState>[];
+      _pathToFiles[path] = files;
+    }
+    files.add(file);
+  }
 }
 
 @visibleForTesting