Add Base64 codec to dart:convert.

tests/lib/lib.status

Index: tests/lib/lib.status
diff --git a/tests/lib/lib.status b/tests/lib/lib.status
index c35ec1d0fe2f2397a227849407469935649e34de..5c2b2fffa58ce6b239bacbea8f686c6b9689514d 100644
--- a/tests/lib/lib.status
+++ b/tests/lib/lib.status
@@ -180,6 +180,7 @@ 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.