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

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

Index: pkg/analyzer/lib/src/context/context.dart
diff --git a/pkg/analyzer/lib/src/context/context.dart b/pkg/analyzer/lib/src/context/context.dart
index 394f30103715469a2c3bdbe5e45806739b53c2cd..7a608a8480722befa74275e4c0167318a597967a 100644
--- a/pkg/analyzer/lib/src/context/context.dart
+++ b/pkg/analyzer/lib/src/context/context.dart
@@ -1011,10 +1011,21 @@ class AnalysisContextImpl implements InternalAnalysisContext {
 
   @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) {
@@ -1744,9 +1755,9 @@ class AnalysisContextImpl implements InternalAnalysisContext {
   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);