Retain elements a finer granularity than library.

dart/tests/compiler/dart2js/memory_compiler.dart

 // Copyright (c) 2013, 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 dart2js.test.memory_compiler;
 
 import 'package:expect/expect.dart';
 import 'memory_source_file_helper.dart';
 
 import '../../../sdk/lib/_internal/compiler/implementation/dart2jslib.dart'
        show NullSink;
 
 import '../../../sdk/lib/_internal/compiler/compiler.dart'
        show DiagnosticHandler;
 
 import 'dart:async';
 
 import '../../../sdk/lib/_internal/compiler/implementation/mirrors/mirrors.dart';
 import '../../../sdk/lib/_internal/compiler/implementation/mirrors/dart2js_mirror.dart';
 
 Compiler compilerFor(Map<String,String> memorySourceFiles,
                      {DiagnosticHandler diagnosticHandler,
-                      List<String> options: const []}) {
+                      List<String> options: const [],
+                      Compiler cachedCompiler}) {
   Uri script = currentDirectory.resolve(nativeToUriPath(Platform.script));
   Uri libraryRoot = script.resolve('../../../sdk/');
   Uri packageRoot = script.resolve('./packages/');
 
   MemorySourceFileProvider.MEMORY_SOURCE_FILES = memorySourceFiles;
   var provider = new MemorySourceFileProvider();
   if (diagnosticHandler == null) {
     diagnosticHandler = new FormattingDiagnosticHandler(provider);
   }
 
   EventSink<String> outputProvider(String name, String extension) {
     if (name != '') throw 'Attempt to output file "$name.$extension"';
     return new NullSink('$name.$extension');
   }
 
   Compiler compiler = new Compiler(provider.readStringFromUri,
                                    outputProvider,
                                    diagnosticHandler,
                                    libraryRoot,
                                    packageRoot,
                                    options);
+  if (cachedCompiler != null) {
+    compiler.coreLibrary = cachedCompiler.libraries['dart:core'];
+    compiler.types = cachedCompiler.types;
+    cachedCompiler.libraries.forEach((String uri, library) {
+      if (library.isPlatformLibrary) {
+        compiler.libraries[uri] = library;
+        compiler.onLibraryScanned(library, library.canonicalUri);
+      }
+    });
+
+    compiler.symbolConstructor = cachedCompiler.symbolConstructor;
+    compiler.mirrorSystemClass = cachedCompiler.mirrorSystemClass;
+    compiler.mirrorsUsedClass = cachedCompiler.mirrorsUsedClass;
+    compiler.mirrorSystemGetNameFunction =
+        cachedCompiler.mirrorSystemGetNameFunction;
+    compiler.symbolImplementationClass =
+        cachedCompiler.symbolImplementationClass;
+    compiler.symbolValidatedConstructor =
+        cachedCompiler.symbolValidatedConstructor;
+    compiler.mirrorsUsedConstructor = cachedCompiler.mirrorsUsedConstructor;
+    compiler.deferredLibraryClass = cachedCompiler.deferredLibraryClass;
+
+    Map cachedTreeElements =
+        cachedCompiler.enqueuer.resolution.resolvedElements;
+    cachedTreeElements.forEach((element, treeElements) {
+      if (element.getLibrary().isPlatformLibrary) {
+        compiler.enqueuer.resolution.resolvedElements[element] =
+            treeElements;
+      }
+    });
+  }
   return compiler;
 }
 
 Future<MirrorSystem> mirrorSystemFor(Map<String,String> memorySourceFiles,
                                      {DiagnosticHandler diagnosticHandler,
                                       List<String> options: const []}) {
   Uri script = currentDirectory.resolve(nativeToUriPath(Platform.script));
   Uri libraryRoot = script.resolve('../../../sdk/');
   Uri packageRoot = script.resolve('./packages/');
 
   MemorySourceFileProvider.MEMORY_SOURCE_FILES = memorySourceFiles;
   var provider = new MemorySourceFileProvider();
   if (diagnosticHandler == null) {
     diagnosticHandler = new FormattingDiagnosticHandler(provider);
   }
 
   List<Uri> libraries = <Uri>[];
   memorySourceFiles.forEach((String path, _) {
     libraries.add(new Uri(scheme: 'memory', path: path));
   });
 
   return analyze(libraries, libraryRoot, packageRoot,
                  provider, diagnosticHandler, options);
 }