cps_ir: add refinement on "success" arguments for a set of whitelisted methods

tests/compiler/dart2js/js_backend_cps_ir.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.
 
 // Test that the CPS IR code generator compiles programs and produces the
 // the expected output.
 
 import 'package:async_helper/async_helper.dart';
 import 'package:expect/expect.dart';
 import 'package:compiler/src/apiimpl.dart' show
@@ skipping 46 matching lines @@
 
   js.Node ast = compiler.enqueuer.codegen.generatedCode[foundElement];
   return js.prettyPrint(ast, compiler).getText();
 }
 
 runTests(List<TestEntry> tests) {
   for (TestEntry test in tests) {
     Map files = {TEST_MAIN_FILE: test.source};
     asyncTest(() async {
       Uri uri = Uri.parse('memory:$TEST_MAIN_FILE');
+      String expected = test.expectation;
+      String found = null;
       try {
         CompilationResult result = await runCompiler(
             entryPoint: uri,
             memorySourceFiles: files,
             options: <String>['--use-cps-ir']);
         Expect.isTrue(result.isSuccess);
         CompilerImpl compiler = result.compiler;
-        String expectation = test.expectation;
-        if (expectation != null) {
-          String expected = test.expectation;
-          String found = test.elementName == null
+        if (expected != null) {
+          found = test.elementName == null
               ? getCodeForMain(compiler)
               : getCodeForMethod(compiler, test.elementName);
-          if (expected != found) {
-            Expect.fail('Expected:\n$expected\nbut found\n$found');
-          }
         }
       } catch (e, st) {
         print(e);
         print(st);
         Expect.fail('The following test failed to compile:\n'
                     '${formatTest(files)}');
       }
+      if (expected != found) {
+        Expect.fail('Unexpected output for test:\n  '
+            '${formatTest(files).replaceAll('\n', '\n  ')}\n'
+            'Expected:\n  ${expected.replaceAll('\n', '\n  ')}\n'
+            'but found:\n  ${found?.replaceAll('\n', '\n  ')}');
+      }
     });
   }
 }