Revert "Remove Expect from core library."

tests/standalone/io/test_extension_tester.dart

Index: tests/standalone/io/test_extension_tester.dart
diff --git a/tests/standalone/io/test_extension_tester.dart b/tests/standalone/io/test_extension_tester.dart
index 707771795f217aba8ffab1b0626e003addadd392..deacbaa5bbfdeb0f28688abf2fef899f61b2cf83 100644
--- a/tests/standalone/io/test_extension_tester.dart
+++ b/tests/standalone/io/test_extension_tester.dart
@@ -6,22 +6,6 @@ library test_extension_test;
 
 import "test_extension.dart";
 
-class Expect {
-  static void equals(expected, actual, [msg]) {
-    if (expected != actual) {
-      if (msg == null) msg = "Expected: $expected. Actual: $actual";
-      throw new RuntimeError(msg);
-    }
-  }
-
-  static void isNull(x, [msg]) {
-    if (x != null) {
-      if (msg != null) msg = "$x not null";
-      throw new RuntimeError(msg);
-    }
-  }
-}
-
 main() {
   Expect.equals('cat 13', new Cat(13).toString(), 'new Cat(13).toString()');