Workaround for accessing part sources while resolving a library.

pkg/analyzer/test/src/dart/analysis/driver_test.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.
 
 library analyzer.test.driver;
 
 import 'dart:async';
 import 'dart:convert';
 
 import 'package:analyzer/dart/ast/ast.dart';
@@ skipping 1145 matching lines @@
 
     driver.addFile(c);
 
     // There is no library which c.dart is a part of, so it has unresolved
     // A and B references.
     AnalysisResult result = await driver.getResult(c);
     expect(result.errors, isNotEmpty);
     expect(result.unit, isNotNull);
   }
 
+  test_part_getResult_overlayOnly() async {
+    var a = _p('/test/lib/a.dart');
+    var b = _p('/test/lib/b.dart');
+    contentOverlay[a] = r'''
+library a;
+part 'b.dart';
+class A {}
+var b = new B();
+''';
+    contentOverlay[b] = 'part of a; class B {}';
+
+    driver.addFile(a);
+    driver.addFile(b);
+
+    AnalysisResult result = await driver.getResult(a);
+    expect(result.errors, isEmpty);
+    expect(_getTopLevelVarType(result.unit, 'b'), 'B');
+  }
+
   test_part_results_afterLibrary() async {
     var a = _p('/test/lib/a.dart');
     var b = _p('/test/lib/b.dart');
     var c = _p('/test/lib/c.dart');
     provider.newFile(
         a,
         r'''
 library a;
 import 'b.dart';
 part 'c.dart';
@@ skipping 404 matching lines @@
   String _p(String path) => provider.convertPath(path);
 
   Future<Null> _waitForIdle() async {
     await idleStatusMonitor.signal;
   }
 
   static String _md5(String content) {
     return hex.encode(md5.convert(UTF8.encode(content)).bytes);
   }
 }