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

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

Index: pkg/analyzer/lib/src/generated/engine.dart
diff --git a/pkg/analyzer/lib/src/generated/engine.dart b/pkg/analyzer/lib/src/generated/engine.dart
index c3aa019682040d09d0ed90af2caeb87aaa47d499..43b9c8af8441f1c7045d6f9ae4284860067bb4fd 100644
--- a/pkg/analyzer/lib/src/generated/engine.dart
+++ b/pkg/analyzer/lib/src/generated/engine.dart
@@ -2023,12 +2023,16 @@ abstract class InternalAnalysisContext implements AnalysisContext {
    * 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