Don't create navigation regions for not existing sources.

pkg/analysis_server/lib/src/computer/computer_navigation.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 computer.navigation;
 
 import 'dart:collection';
 
 import 'package:analysis_server/src/protocol_server.dart' as protocol;
 import 'package:analyzer/src/generated/ast.dart';
 import 'package:analyzer/src/generated/element.dart';
 import 'package:analyzer/src/generated/scanner.dart';
+import 'package:analyzer/src/generated/source.dart';
 
 /**
  * A computer for navigation regions in a Dart [CompilationUnit].
  */
 class DartUnitNavigationComputer {
   final CompilationUnit _unit;
 
   final List<String> files = <String>[];
   final Map<String, int> fileMap = new HashMap<String, int>();
   final List<protocol.NavigationTarget> targets = <protocol.NavigationTarget>[];
@@ skipping 131 matching lines @@
     } else if (parent is ConstructorDeclaration &&
         parent.redirectedConstructor == node) {
       _addConstructorName(node, node);
     }
   }
 
   @override
   visitExportDirective(ExportDirective node) {
     ExportElement exportElement = node.element;
     if (exportElement != null) {
-      Element element = exportElement.exportedLibrary;
-      computer._addRegion_tokenStart_nodeEnd(node.keyword, node.uri, element);
+      Element libraryElement = exportElement.exportedLibrary;
+      _addUriDirectiveRegion(node, libraryElement);
     }
     super.visitExportDirective(node);
   }
 
   @override
   visitImportDirective(ImportDirective node) {
     ImportElement importElement = node.element;
     if (importElement != null) {
-      Element element = importElement.importedLibrary;
-      computer._addRegion_tokenStart_nodeEnd(node.keyword, node.uri, element);
+      Element libraryElement = importElement.importedLibrary;
+      _addUriDirectiveRegion(node, libraryElement);
     }
     super.visitImportDirective(node);
   }
 
   @override
   visitIndexExpression(IndexExpression node) {
     super.visitIndexExpression(node);
     computer._addRegionForToken(node.rightBracket, node.bestElement);
   }
 
   @override
   visitPartDirective(PartDirective node) {
-    computer._addRegion_tokenStart_nodeEnd(
-        node.keyword, node.uri, node.element);
+    _addUriDirectiveRegion(node, node.element);
     super.visitPartDirective(node);
   }
 
   @override
   visitPartOfDirective(PartOfDirective node) {
     computer._addRegion_tokenStart_nodeEnd(
         node.keyword, node.libraryName, node.element);
     super.visitPartOfDirective(node);
   }
 
@@ skipping 53 matching lines @@
       computer._addRegion_nodeStart_nodeEnd(parent, typeName.name, element);
       // <TypeA, TypeB>
       typeArguments.accept(this);
       // optional ".name"
       if (node.period != null) {
         computer._addRegion_tokenStart_nodeEnd(node.period, node, element);
       }
     }
   }
 
+  /**
+   * If the source of the given [element] (referenced by the [node]) exists,
+   * then add the navigation region from the [node] to the [element].
+   */
+  void _addUriDirectiveRegion(UriBasedDirective node, Element element) {
+    if (element != null) {
+      Source source = element.source;
+      if (element.context.exists(source)) {
+        computer._addRegion_tokenStart_nodeEnd(node.keyword, node.uri, element);
+      }
+    }
+  }
+
   void _safelyVisit(AstNode node) {
     if (node != null) {
       node.accept(this);
     }
   }
 }