Stop defensively copying AST structures before resolution

pkg/analyzer/test/src/context/context_test.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 test.src.context.context_test;
 
 import 'dart:async';
 
 import 'package:analyzer/file_system/memory_file_system.dart';
 import 'package:analyzer/src/cancelable_future.dart';
@@ skipping 1482 matching lines @@
     expect(context.getResolvedCompilationUnit2(partSource, libSource), isNull,
         reason: "part resolved 2");
     // update and analyze #2
     context.setContents(libSource, "library lib; part 'test-part.dart';");
     expect(context.getResolvedCompilationUnit2(libSource, libSource), isNull,
         reason: "library changed 3");
     expect(context.getResolvedCompilationUnit2(partSource, libSource), isNull,
         reason: "part changed 3");
     _analyzeAll_assertFinished();
     expect(context.getResolvedCompilationUnit2(libSource, libSource), isNotNull,
-        reason: "library resolved 2");
+        reason: "library resolved 3");
     expect(
         context.getResolvedCompilationUnit2(partSource, libSource), isNotNull,
         reason: "part resolved 3");
   }
 
   void test_performAnalysisTask_changeLibraryThenPartContents() {
     Source libSource =
         addSource("/test.dart", "library lib; part 'test-part.dart';");
     Source partSource = addSource("/test-part.dart", "part of lib;");
     _analyzeAll_assertFinished();
@@ skipping 1001 matching lines @@
   }
 }
 
 class _AnalysisContextImplTest_test_applyChanges_removeContainer
     implements SourceContainer {
   Source libB;
   _AnalysisContextImplTest_test_applyChanges_removeContainer(this.libB);
   @override
   bool contains(Source source) => source == libB;
 }