Fix issue where errors aren't reported for a new overlay

pkg/analyzer/lib/src/generated/engine.dart

 // Copyright (c) 2014, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 library analyzer.src.generated.engine;
 
 import 'dart:async';
 import 'dart:collection';
 
 import 'package:analyzer/dart/ast/ast.dart';
@@ skipping 2005 matching lines @@
    * [originalContents] is the former contents of the file, and [newContents]
    * is the updated contents.  If [notify] is true, a source changed event is
    * triggered.
    *
    * Normally it should not be necessary for clients to call this function,
    * since it will be automatically invoked in response to a call to
    * [applyChanges] or [setContents].  However, if this analysis context is
    * sharing its content cache with other contexts, then the client must
    * manually update the content cache and call this function for each context.
    *
-   * Return `true` if the change was significant to this context (i.e. [source]
-   * is either implicitly or explicitly analyzed by this context, and a change
-   * actually occurred).
+   * If [explicit] is true and newContents is not null, unconditionally add
+   * as an explicit source and return true.
+   *
+   * Otherwise, return `true` if the change was significant to this context
+   * (i.e. [source] was already either implicitly or explicitly analyzed by
+   * this context, and a change actually occurred).
    */
   bool handleContentsChanged(
-      Source source, String originalContents, String newContents, bool notify);
+      Source source, String originalContents, String newContents, bool notify,
+      {bool explicit: false});
 
   /**
    * Given an [elementMap] mapping the source for the libraries represented by
    * the corresponding elements to the elements representing the libraries,
    * record those mappings.
    */
   void recordLibraryElements(Map<Source, LibraryElement> elementMap);
 
   /**
    * Return `true` if errors should be produced for the given [source].
@@ skipping 433 matching lines @@
    * The data that was created from the source.
    */
   final E data;
 
   /**
    * Initialize a newly created holder to associate the given [data] with the
    * given [modificationTime].
    */
   TimestampedData(this.modificationTime, this.data);
 }