Revert "Remove Expect from core library."

tests/standalone/io/test_extension_fail_test.dart

 // Copyright (c) 2012, 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.
 //
 // Dart test program for testing native extensions.
 
 import "dart:async";
 import "dart:io";
 
 Future copyFileToDirectory(Path file, Path directory) {
   String src = file.toNativePath();
   String dst = directory.toNativePath();
   switch (Platform.operatingSystem) {
     case 'linux':
     case 'macos':
       return Process.run('cp', [src, dst]);
     case 'windows':
       return Process.run('cmd.exe', ['/C', 'copy $src $dst']);
     default:
-      throw new RuntimeError(
-          'Unknown operating system ${Platform.operatingSystem}');
+      Expect.fail('Unknown operating system ${Platform.operatingSystem}');
   }
 }
 
 Path getExtensionPath(Path buildDirectory) {
   switch (Platform.operatingSystem) {
     case 'linux':
       return buildDirectory.append('lib.target/libtest_extension.so');
     case 'macos':
       return buildDirectory.append('libtest_extension.dylib');
     case 'windows':
       return buildDirectory.append('test_extension.dll');
     default:
-      throw new RuntimeError(
-          'Unknown operating system ${Platform.operatingSystem}');
+      Expect.fail('Unknown operating system ${Platform.operatingSystem}');
   }
 }
 
 void main() {
   Options options = new Options();
 
   Path scriptDirectory = new Path(options.script).directoryPath;
   Path buildDirectory = new Path(options.executable).directoryPath;
   Directory tempDirectory = new Directory('').createTempSync();
   Path testDirectory = new Path(tempDirectory.path);
 
   // Copy test_extension shared library, test_extension.dart and
   // test_extension_fail_tester.dart to the temporary test directory.
   copyFileToDirectory(getExtensionPath(buildDirectory),
                       testDirectory).then((_) {
     Path extensionDartFile = scriptDirectory.append('test_extension.dart');
     return copyFileToDirectory(extensionDartFile, testDirectory);
   }).then((_) {
     Path testExtensionTesterFile =
         scriptDirectory.append('test_extension_fail_tester.dart');
     return copyFileToDirectory(testExtensionTesterFile, testDirectory);
   }).then((_) {
     Path script = testDirectory.append('test_extension_fail_tester.dart');
     return Process.run(options.executable, [script.toNativePath()]);
   }).then((ProcessResult result) {
     print("ERR: ${result.stderr}\n\n");
     print("OUT: ${result.stdout}\n\n");
-    if (result.exitCode != 255) {
-      throw new RuntimeError("bad exit code");
-    }
-    if (!result.stderr.contains("Unhandled exception:")) {
-      throw new RuntimeError("stderr doesn't contain unhandled exception.");
-    }
-    if (!result.stderr.contains("ball")) {
-      throw new RuntimeError("stderr doesn't contain 'ball'.");
-    }
+    Expect.equals(255, result.exitCode);
+    Expect.isTrue(result.stderr.contains("Unhandled exception:"));
+    Expect.isTrue(result.stderr.contains("ball"));
   }).whenComplete(() => tempDirectory.deleteSync(recursive: true));
 }