Make FileByteStore accept a unique temporary name suffix.

pkg/front_end/lib/src/byte_store/file_byte_store.dart

Index: pkg/front_end/lib/src/byte_store/file_byte_store.dart
diff --git a/pkg/front_end/lib/src/byte_store/file_byte_store.dart b/pkg/front_end/lib/src/byte_store/file_byte_store.dart
index 376f65bf7de007edf1f63fe27a4339c9cb8b615e..10769f598c049cd0a6aa2cb4dcb8d75ba836f62d 100644
--- a/pkg/front_end/lib/src/byte_store/file_byte_store.dart
+++ b/pkg/front_end/lib/src/byte_store/file_byte_store.dart
@@ -142,9 +142,14 @@ class EvictingFileByteStore implements ByteStore {
  */
 class FileByteStore implements ByteStore {
   final String _cachePath;
-  final String _tempName = 'temp_$pid';
+  final String _tempName;
 
-  FileByteStore(this._cachePath);
+  /**
+   * If the same cache path is used from more than one isolate of the same
+   * process, then a unique [tempNameSuffix] must be provided for each isolate.
+   */
+  FileByteStore(this._cachePath, {String tempNameSuffix: ''})
+      : _tempName = 'temp_${pid}_${tempNameSuffix}';
 
   @override
   List<int> get(String key) {