Use analyzeOnly in kernel/impact_test.

tests/compiler/dart2js/kernel/compiler_helper.dart

 // Copyright (c) 2017, 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.
 
 // Partial test that the closed world computed from [WorldImpact]s derived from
 // kernel is equivalent to the original computed from resolution.
 library dart2js.kernel.compiler_helper;
 
 import 'dart:async';
 import 'dart:io';
@@ skipping 91 matching lines @@
     print('---- analyze-all -------------------------------------------------');
   }
   Compiler compiler = compilerFor(
       entryPoint: entryPoint,
       memorySourceFiles: memorySourceFiles,
       options: [
         Flags.analyzeAll,
         Flags.useKernelInSsa,
         Flags.enableAssertMessage
       ]);
+  compiler.resolution.retainCachesForTesting = true;
   await compiler.run(entryPoint);
 
   if (printSteps) {
     print('---- closed world from kernel ------------------------------------');
   }
   Compiler compiler2 = compilerFor(
       entryPoint: entryPoint,
       memorySourceFiles: memorySourceFiles,
       options: [Flags.analyzeOnly, Flags.enableAssertMessage, Flags.useKernel]);
   ElementResolutionWorldBuilder.useInstantiationMap = true;
@@ skipping 73 matching lines @@
       options: [Flags.useKernel]..addAll(options),
       outputProvider: compilerOutput);
   ElementResolutionWorldBuilder.useInstantiationMap = true;
   compiler.resolution.retainCachesForTesting = true;
   if (beforeRun != null) {
     beforeRun(compiler);
   }
   await compiler.run(dillFile);
   return compiler;
 }