Implement SearchEngine for the new driver.

pkg/analyzer/lib/src/dart/analysis/search.dart

 // Copyright (c) 2016, 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.
 
 import 'dart:async';
 
 import 'package:analyzer/dart/ast/ast.dart';
 import 'package:analyzer/dart/ast/visitor.dart';
 import 'package:analyzer/dart/element/element.dart';
 import 'package:analyzer/dart/element/visitor.dart';
@@ skipping 21 matching lines @@
     if (kind == ElementKind.LABEL || kind == ElementKind.LOCAL_VARIABLE) {
       return _searchReferences_Local(element, (n) => n is Block);
     }
     // TODO(scheglov) support other kinds
     return const <SearchResult>[];
   }
 
   Future<List<SearchResult>> _searchReferences_Local(
       Element element, bool isRootNode(AstNode n)) async {
     String path = element.source.fullName;
+    if (!_driver.addedFiles.contains(path)) {
+      return const <SearchResult>[];
+    }
 
     // Prepare the unit.
     AnalysisResult analysisResult = await _driver.getResult(path);
     CompilationUnit unit = analysisResult.unit;
     if (unit == null) {
       return const <SearchResult>[];
     }
 
     // Prepare the node.
     AstNode node = new NodeLocator(element.nameOffset).searchWithin(unit);
@@ skipping 145 matching lines @@
   }
 
   void _addResult(AstNode node, SearchResultKind kind) {
     bool isQualified = node.parent is Label;
     var finder = new _ContainingElementFinder(node.offset);
     enclosingUnitElement.accept(finder);
     results.add(new SearchResult._(element, finder.containingElement, kind,
         node.offset, node.length, true, isQualified));
   }
 }