Roll Dart to fix the build.

Change-Id: I4d338cb20c70c8ce263e7f60a177fd7e17052075
diff --git a/runtimes/dart b/runtimes/dart
index 9216314..e5c3352 100644
--- a/runtimes/dart
+++ b/runtimes/dart
@@ -9,7 +9,7 @@
              path="third_party/dart"
              remote="https://dart.googlesource.com/sdk"
              gerrithost="https://dart-review.googlesource.com"
-             revision="ec8dfb9b534de8f25e084d1e0336bd5815741c7f"/>
+             revision="68a4c5783d5fef10a512efff4c6d4f9842066ba5"/>
     <project name="dart/observatory_pub_packages"
              path="third_party/dart/third_party/observatory_pub_packages"
              remote="https://dart.googlesource.com/observatory_pub_packages/"