Add Base64 codec to dart:convert.

pkg/expect/lib/expect.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.
 
 /**
  * This library contains an Expect class with static methods that can be used
  * for simple unit-tests.
  */
 library expect;
 
@@ skipping 339 matching lines @@
 
   /**
    * Calls the function [f] and verifies that it throws an exception.
    * The optional [check] function can provide additional validation
    * that the correct exception is being thrown.  For example, to check
    * the type of the exception you could write this:
    *
    *     Expect.throws(myThrowingFunction, (e) => e is MyException);
    */
   static void throws(void f(),
-                     [_CheckExceptionFn check = null,
+                     [bool check(exception) = null,
                       String reason = null]) {
     String msg = reason == null ? "" : "($reason)";
     if (f is! _Nullary) {
       // Only throws from executing the funtion body should count as throwing.
       // The failure to even call `f` should throw outside the try/catch.
       _fail("Expect.throws$msg: Function f not callable with zero arguments");
     }
     try {
       f();
     } catch (e, s) {
@@ skipping 10 matching lines @@
   static String _getMessage(String reason)
       => (reason == null) ? "" : ", '$reason'";
 
   static void _fail(String message) {
     throw new ExpectException(message);
   }
 }
 
 bool _identical(a, b) => identical(a, b);
 
-typedef bool _CheckExceptionFn(exception);
 typedef _Nullary();  // Expect.throws argument must be this type.
 
 class ExpectException implements Exception {
   ExpectException(this.message);
   String toString() => message;
   String message;
 }
 
 /// Annotation class for testing of dart2js. Use this as metadata on method
 /// declarations to disable inlining of the annotated method.
@@ skipping 15 matching lines @@
   const TrustTypeAnnotations();
 }
 
 /// Annotation class for testing of dart2js. Use this as metadata on method
 /// declarations to disable closed world assumptions on parameters, effectively
 /// assuming that the runtime arguments could be any value. Note that the
 /// constraints due to [TrustTypeAnnotations] still apply.
 class AssumeDynamic {
   const AssumeDynamic();
 }