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

pkg/analysis_server/test/analysis/update_content_test.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 test.analysis.updateContent;
 
 import 'package:analysis_server/plugin/protocol/protocol.dart';
 import 'package:analysis_server/src/analysis_server.dart';
 import 'package:analysis_server/src/constants.dart';
 import 'package:analysis_server/src/services/index/index.dart';
@@ skipping 24 matching lines @@
   int navigationCount = 0;
 
   Index createIndex() {
     return new _MockIndex();
   }
 
   @override
   void processNotification(Notification notification) {
     if (notification.event == ANALYSIS_ERRORS) {
       var decoded = new AnalysisErrorsParams.fromNotification(notification);
-      filesErrors[decoded.file] = decoded.errors;
+      _format(AnalysisError e) => "${e.location.startLine}: ${e.message}";
+      filesErrors[decoded.file] = decoded.errors.map(_format).toList();
     }
     if (notification.event == ANALYSIS_NAVIGATION) {
       navigationCount++;
     }
     if (notification.event == SERVER_ERROR) {
       serverErrorCount++;
     }
   }
 
   test_discardNotifications_onSourceChange() async {
@@ skipping 125 matching lines @@
       expect(sources, hasLength(1));
       expect(sources[0].fullName, filePath);
     }
     expect(_getUserSources(context2), isEmpty);
     // remove the overlay - no sources
     server.updateContent('2', {filePath: new RemoveContentOverlay()});
     expect(_getUserSources(context1), isEmpty);
     expect(_getUserSources(context2), isEmpty);
   }
 
+  test_overlay_addPreviouslyImported() async {
+    Folder project = resourceProvider.newFolder('/project');
+    handleSuccessfulRequest(
+        new AnalysisSetAnalysisRootsParams([project.path], []).toRequest('0'));
+
+    server.updateContent('1',
+        {'/project/main.dart': new AddContentOverlay('import "target.dart";')});
+    await server.onAnalysisComplete;
+    expect(filesErrors, {
+      '/project/main.dart': ["1: Target of URI does not exist: 'target.dart'"],
+      '/project/target.dart': []
+    });
+
+    server.updateContent('1',
+        {'/project/target.dart': new AddContentOverlay('import "none.dart";')});
+    await server.onAnalysisComplete;
+    expect(filesErrors, {
+      '/project/main.dart': ["1: Unused import"],
+      '/project/target.dart': ["1: Target of URI does not exist: 'none.dart'"],
+      '/project/none.dart': []
+    });
+  }
+
   test_removeOverlay_incrementalChange() async {
     createProject();
     addTestFile('main() { print(1); }');
     await server.onAnalysisComplete;
     CompilationUnit unit = _getTestUnit();
     // add an overlay
     server.updateContent(
         '1', {testFile: new AddContentOverlay('main() { print(2); }')});
     // it was an incremental change
     await server.onAnalysisComplete;
@@ skipping 65 matching lines @@
 
   _ArgumentMatcher_CompilationUnit(this.file);
 
   @override
   bool matches(arg) {
     return arg is CompilationUnit && arg.element.source.fullName == file;
   }
 }
 
 class _MockIndex extends TypedMock implements Index {}