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

pkg/analysis_server/lib/src/analysis_server.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 analysis.server;
 
 import 'dart:async';
 import 'dart:collection';
 import 'dart:core' hide Resource;
 import 'dart:math' show max;
@@ skipping 1252 matching lines @@
           throw new RequestFailure(new Response(id,
               error: new RequestError(RequestErrorCode.INVALID_OVERLAY_CHANGE,
                   'Invalid overlay change')));
         }
       } else if (change is RemoveContentOverlay) {
         newContents = null;
       } else {
         // Protocol parsing should have ensured that we never get here.
         throw new AnalysisException('Illegal change type');
       }
+
+      AnalysisContext containingContext = getContainingContext(file);
+
+      // Check for an implicitly added but missing source.
+      // (For example, the target of an import might not exist yet.)
+      // We need to do this before setContents, which changes the stamp.
+      bool wasMissing = containingContext?.getModificationStamp(source) == -1;
+
       overlayState.setContents(source, newContents);
       // If the source does not exist, then it was an overlay-only one.
       // Remove it from contexts.
       if (newContents == null && !source.exists()) {
         for (InternalAnalysisContext context in analysisContexts) {
           List<Source> sources = context.getSourcesWithFullName(file);
           ChangeSet changeSet = new ChangeSet();
           sources.forEach(changeSet.removedSource);
           context.applyChanges(changeSet);
           schedulePerformAnalysisOperation(context);
         }
         return;
       }
       // Update all contexts.
       bool anyContextUpdated = false;
       for (InternalAnalysisContext context in analysisContexts) {
         List<Source> sources = context.getSourcesWithFullName(file);
         sources.forEach((Source source) {
           anyContextUpdated = true;
+          if (context == containingContext && wasMissing) {
+            // Promote missing source to an explicitly added Source.
+            context.applyChanges(new ChangeSet()..addedSource(source));
+            schedulePerformAnalysisOperation(context);
+          }
           if (context.handleContentsChanged(
               source, oldContents, newContents, true)) {
             schedulePerformAnalysisOperation(context);
           } else {
             // When the client sends any change for a source, we should resend
             // subscribed notifications, even if there were no changes in the
             // source contents.
             // TODO(scheglov) consider checking if there are subscriptions.
             if (AnalysisEngine.isDartFileName(file)) {
               List<CompilationUnit> dartUnits =
@@ skipping 401 matching lines @@
   /**
    * The [PerformanceTag] for time spent in server request handlers.
    */
   static PerformanceTag serverRequests = new PerformanceTag('serverRequests');
 
   /**
    * The [PerformanceTag] for time spent in split store microtasks.
    */
   static PerformanceTag splitStore = new PerformanceTag('splitStore');
 }