Improve how we report unittest failures by stripping out the commented input code

tests/compiler/dart2js/cps_ir/runner.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.
 
 // Test that the CPS IR code generator compiles programs and produces the
 // the expected output.
 
 import 'dart:io';
 
 import 'package:async_helper/async_helper.dart';
@@ skipping 10 matching lines @@
 // test output. By default we match the output of main.
 final RegExp elementNameRegExp = new RegExp(r'^// Method to test: (.*)$',
     multiLine: true);
 
 runTest(String filename, {bool update: false}) {
   var outputname = filename.replaceFirst('.dart', '.js');
   String source = new File.fromUri(Platform.script.resolve('input/$filename'))
       .readAsStringSync();
   var expectedFile =
       new File.fromUri(Platform.script.resolve('expected/$outputname'));
-  String expected = expectedFile.existsSync()
-    ? expectedFile.readAsStringSync() : '';
+  String expected = '';
+  if (expectedFile.existsSync()) {
+    expected = expectedFile.readAsStringSync()
+      .replaceAll(new RegExp('^//.*\n', multiLine: true), '')
+      .trim();
+  }
+
   var match = elementNameRegExp.firstMatch(source);
   var elementName = match?.group(1);
 
   Map files = {
       TEST_MAIN_FILE: source,
       'package:expect/expect.dart': '''
           class NoInline {
             const NoInline();
           }
           class TrustTypeAnnotations {
             const TrustTypeAnnotations();
           }
           class AssumeDynamic {
             const AssumeDynamic();
           }
        ''',
    };
   asyncTest(() async {
     Uri uri = Uri.parse('memory:$TEST_MAIN_FILE');
     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;
       if (expected != null) {
-        String output = elementName == null
+        found = elementName == null
             ? _getCodeForMain(compiler)
             : _getCodeForMethod(compiler, elementName);
-        // Include the input in a comment of the expected file to make it easier
-        // to see the relation between input and output in code reviews.
-        found = '// Expectation for test: \n'
-            '// ${source.trim().replaceAll('\n', '\n// ')}\n\n'
-            '$output\n';
       }
     } catch (e, st) {
       print(e);
       print(st);
       var message = 'The following test failed to compile:\n'
                     '${_formatTest(files)}';
       if (update) {
         print('\n\n$message\n');
         return;
       } else {
         Expect.fail(message);
       }
     }
     if (expected != found) {
       if (update) {
-        expectedFile.writeAsStringSync(found);
+        // Include the input in a comment of the expected file to make it easier
+        // to see the relation between input and output in code reviews.
+        String comment = source.trim().replaceAll('\n', '\n// ');
+        expectedFile.writeAsStringSync('// Expectation for test: \n'
+            '// ${comment}\n\n${found}\n');
         print('INFO: $expectedFile was updated');
       } else {
         Expect.fail('Unexpected output for test:\n  '
             '${_formatTest(files).replaceAll('\n', '\n  ')}\n'
-            'Expected:\n  ${expected.replaceAll('\n', '\n  ')}\n'
+            'Expected:\n  ${expected.replaceAll('\n', '\n  ')}\n\n'
             'but found:\n  ${found?.replaceAll('\n', '\n  ')}\n'
             '$regenerateCommand');
       }
     }
   });
 }
 
 String get regenerateCommand {
   var flags = Platform.packageRoot == null
     ? '' : '--package-root=${Platform.packageRoot} ';
@@ skipping 33 matching lines @@
     }
   }
 
   if (foundElement == null) {
     Expect.fail('There is no compiled element called $name');
   }
 
   js.Node ast = compiler.enqueuer.codegen.generatedCode[foundElement];
   return js.prettyPrint(ast, compiler);
 }