Roll dart dependency to dev.20 (flutter#125)

Author: aam

tools/dart/update.py

@@ -19,7 +19,7 @@
 # How to roll the dart sdk: Just change this url! We write this to the stamp
 # file after we download, and then check the stamp file for differences.
 SDK_URL_BASE = ('http://gsdview.appspot.com/dart-archive/channels/dev/raw/'
-                '2.0.0-dev.16.0/sdk/')
+                '2.0.0-dev.20.0/sdk/')
 
 LINUX_64_SDK = 'dartsdk-linux-x64-release.zip'
 MACOS_64_SDK = 'dartsdk-macos-x64-release.zip'