fix import uri completion exception - fixes #24709

pkg/analysis_server/lib/src/services/completion/uri_contributor.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 services.completion.contributor.dart.importuri;
 
 import 'dart:async';
 import 'dart:core' hide Resource;
 
 import 'package:analysis_server/src/services/completion/dart_completion_manager.dart';
@@ skipping 56 matching lines @@
         // and there is no semicolon
         visitSimpleStringLiteral(uri);
       }
     }
   }
 
   @override
   visitSimpleStringLiteral(SimpleStringLiteral node) {
     AstNode parent = node.parent;
     if (parent is NamespaceDirective && parent.uri == node) {
-      String partial = node.literal.lexeme.substring(
-          node.contentsOffset - node.offset, request.offset - node.offset);
-      request.replacementOffset = node.contentsOffset;
-      request.replacementLength = node.contentsEnd - node.contentsOffset;
-      _addDartSuggestions();
-      _addPackageSuggestions(partial);
-      _addFileSuggestions(partial);
+      String partialUri = _extractPartialUri(node);
+      if (partialUri != null) {
+        _addDartSuggestions();
+        _addPackageSuggestions(partialUri);
+        _addFileSuggestions(partialUri);
+      }
     } else if (parent is PartDirective && parent.uri == node) {
-      String partial = node.literal.lexeme.substring(
-          node.contentsOffset - node.offset, request.offset - node.offset);
-      request.replacementOffset = node.contentsOffset;
-      request.replacementLength = node.contentsEnd - node.contentsOffset;
-      _addFileSuggestions(partial);
+      String partialUri = _extractPartialUri(node);
+      if (partialUri != null) {
+        _addFileSuggestions(partialUri);
+      }
     }
   }
 
   void _addDartSuggestions() {
     _addSuggestion('dart:');
     SourceFactory factory = request.context.sourceFactory;
     for (SdkLibrary lib in factory.dartSdk.sdkLibraries) {
       if (!lib.isInternal && !lib.isImplementation) {
         if (!lib.shortName.startsWith('dart:_')) {
           _addSuggestion(lib.shortName,
@@ skipping 83 matching lines @@
       {int relevance: DART_RELEVANCE_DEFAULT}) {
     request.addSuggestion(new CompletionSuggestion(
         CompletionSuggestionKind.IMPORT,
         relevance,
         completion,
         completion.length,
         0,
         false,
         false));
   }
+
+  String _extractPartialUri(SimpleStringLiteral node) {
+    if (request.offset < node.contentsOffset) {
+      return null;
+    }
+    String partial = node.literal.lexeme.substring(
+        node.contentsOffset - node.offset, request.offset - node.offset);
+    request.replacementOffset = node.contentsOffset;
+    request.replacementLength = node.contentsEnd - node.contentsOffset;
+    return partial;
+  }
 }