Don't create navigation regions for not existing sources.

pkg/analysis_server/lib/src/computer/computer_navigation.dart

Index: pkg/analysis_server/lib/src/computer/computer_navigation.dart
diff --git a/pkg/analysis_server/lib/src/computer/computer_navigation.dart b/pkg/analysis_server/lib/src/computer/computer_navigation.dart
index 2236ee4d5cf0cb912c4e3e92d405f450e14f24cd..504bf407323b1315f48b1073cef074be82fb5989 100644
--- a/pkg/analysis_server/lib/src/computer/computer_navigation.dart
+++ b/pkg/analysis_server/lib/src/computer/computer_navigation.dart
@@ -10,6 +10,7 @@ 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].
@@ -161,8 +162,8 @@ class _DartUnitNavigationComputerVisitor extends RecursiveAstVisitor {
   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);
   }
@@ -171,8 +172,8 @@ class _DartUnitNavigationComputerVisitor extends RecursiveAstVisitor {
   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);
   }
@@ -185,8 +186,7 @@ class _DartUnitNavigationComputerVisitor extends RecursiveAstVisitor {
 
   @override
   visitPartDirective(PartDirective node) {
-    computer._addRegion_tokenStart_nodeEnd(
-        node.keyword, node.uri, node.element);
+    _addUriDirectiveRegion(node, node.element);
     super.visitPartDirective(node);
   }
 
@@ -260,6 +260,19 @@ class _DartUnitNavigationComputerVisitor extends RecursiveAstVisitor {
     }
   }
 
+  /**
+   * 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);