Add Base64 codec to dart:convert.

tests/lib/lib.status

 # 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.
 
 [ $unchecked ]
 mirrors/redirecting_factory_different_type_test: SkipByDesign # Tests type checks.
 
 [ $compiler == dart2js ]
 async/schedule_microtask6_test: RuntimeError # global error handling is not supported. Issue 5958
 
@@ skipping 162 matching lines @@
 async/future_constructor2_test: Fail # Timer interface not supported: Issue 7728.
 mirrors/mirrors_reader_test: Skip # Running in v8 suffices. Issue 16589 - RuntimeError.  Issue 22130 - Crash (out of memory).
 
 [ $compiler == dart2js && $checked ]
 convert/utf85_test: Pass, Slow # Issue 12029.
 mirrors/list_constructor_test/01: RuntimeError # Issue 19635.
 
 [ $compiler == dart2js ]
 convert/chunked_conversion_utf88_test: Slow, Pass
 convert/utf85_test: Slow, Pass
+convert/base64_test/01: Fail, OK # Uses bit-wise operations to detect invalid values. Some large invalid values accepted by dart2js.
 mirrors/globalized_closures_test/00: RuntimeError # Issue 17118. Please remove the multi-test comments when this test starts succeeding.
 mirrors/globalized_closures2_test/00: RuntimeError # Issue 17118. Please remove the multi-test comments when this test starts succeeding.
 
 
 [ $compiler == dart2js && ( $browser || $runtime == d8 ) ]
 async/timer_not_available_test: Fail, OK # only meant to test when there is no way to
                                          # implement timer (currently only in jsshell)
 
 # 'js' tests import the dart:js library, so they only make sense in
 # a browser environment.
@@ skipping 146 matching lines @@
 convert/json_pretty_test: RuntimeError # Please triage this failure.
 convert/line_splitter_test: RuntimeError # Please triage this failure.
 mirrors/deferred_mirrors_metadata_test: RuntimeError # U.loadLibrary is not a function
 mirrors/deferred_mirrors_metatarget_test: RuntimeError # X.loadLibrary is not a function
 mirrors/deferred_mirrors_update_test: RuntimeError # U.loadLibrary is not a function
 mirrors/delegate_call_through_getter_test: RuntimeError # Please triage this failure.
 mirrors/delegate_test: RuntimeError # Please triage this failure.
 mirrors/library_enumeration_deferred_loading_test: RuntimeError # L.loadLibrary is not a function
 mirrors/symbol_validation_test/none: RuntimeError # Please triage this failure.
 mirrors/typedef_deferred_library_test: RuntimeError # G.loadLibrary is not a function