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

pkg/analyzer/lib/src/context/context.dart

 // Copyright (c) 2015, 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.context.context;
 
 import 'dart:async';
 import 'dart:collection';
 
 import 'package:analyzer/dart/ast/ast.dart';
@@ skipping 993 matching lines @@
     return _cache.getValue(target, result);
   }
 
   @override
   List<Source> getSourcesWithFullName(String path) {
     return analysisCache.getSourcesWithFullName(path);
   }
 
   @override
   bool handleContentsChanged(
-      Source source, String originalContents, String newContents, bool notify) {
+      Source source, String originalContents, String newContents, bool notify,
+      {bool explicit: false}) {
+    explicit = explicit && (newContents != null);
     CacheEntry entry = _cache.get(source);
     if (entry == null) {
-      return false;
+      if (explicit) {
+        // Create it now.
+        entry = _createCacheEntry(source, true);
+      } else {
+        return false;
+      }
+    }
+    if (explicit) {
+      // Mark as explicit whether there was any other change or not.
+      entry.explicitlyAdded = true;
     }
     bool changed = newContents != originalContents;
     if (newContents != null) {
       if (changed) {
         if (!analysisOptions.incremental ||
             !_tryPoorMansIncrementalResolution(source, newContents)) {
           // Don't compare with old contents because the cache has already been
           // updated, and we know at this point that it changed.
           _sourceChanged(source, compareWithOld: false);
         }
@@ skipping 709 matching lines @@
     }
   }
 
   /**
    * Create an entry for the newly added [source] and invalidate any sources
    * that referenced the source before it existed.
    */
   void _sourceAvailable(Source source) {
     driver.reset();
     // TODO(brianwilkerson) This method needs to check whether the source was
-    // previously being implicitly analyzed. If so, the cache entry needs to be
-    // update to reflect the new status and an event needs to be generated to
-    // inform clients that it is no longer being implicitly analyzed.
+    // previously being implicitly analyzed. If so, an event needs to be
+    // generated to inform clients that it is no longer being implicitly
+    // analyzed.
     CacheEntry entry = _cache.get(source);
     if (entry == null) {
       _createCacheEntry(source, true);
     } else {
       entry.explicitlyAdded = true;
       entry.modificationTime = getModificationStamp(source);
       entry.setState(CONTENT, CacheState.INVALID);
     }
   }
 
@@ skipping 383 matching lines @@
     }
     DartSdk sdk = factory.dartSdk;
     if (sdk == null) {
       throw new IllegalArgumentException(
           "The source factory for an SDK analysis context must have a DartUriResolver");
     }
     return new AnalysisCache(
         <CachePartition>[AnalysisEngine.instance.partitionManager.forSdk(sdk)]);
   }
 }