Roll dart deps (flutter#4644)

* Roll dart deps

* Remove pin

* Roll buildroot to dart dev.20

.travis.yml

@@ -1,7 +1,6 @@
 language: dart
 dart:
-  # Pin to dev.16 to overcome dartbug.com/31940
-  - dev/release/2.0.0-dev.16.0
+  - dev
 sudo: false
 before_script:
   - ./travis/setup.sh

DEPS

@@ -34,7 +34,7 @@ vars = {
   'dart_revision': '32396138ff733cbe8d9ac32178e6cffccdf80644',
 
   'dart_args_tag': '0.13.7',
-  'dart_async_tag': '2.0.2',
+  'dart_async_tag': 'corelib_2_2_1',
   'dart_barback_tag': '0.15.2+14',
   'dart_bazel_worker_tag': 'v0.1.9',
   'dart_boolean_selector_tag': '1.0.2',
@@ -64,7 +64,7 @@ vars = {
   'dart_logging_tag': '0.11.3+1',
   'dart_markdown_tag': '1.0.0',
   'dart_matcher_tag': '0.12.1+4',
-  'dart_mime_tag': '0.9.4',
+  'dart_mime_tag': '0.9.6',
   'dart_mockito_tag': '2.0.2',
   'dart_mustache4dart_tag': 'v2.1.0',
   'dart_oauth2_tag': '1.1.0',
@@ -94,9 +94,9 @@ vars = {
   'dart_tuple_tag': 'v1.0.1',
   'dart_typed_data_tag': '1.1.3',
   'dart_usage_tag': '3.3.0',
-  'dart_utf_tag': '0.9.0+3',
-  'dart_watcher_tag': '0.9.7+4',
-  'dart_web_socket_channel_tag': '1.0.6',
+  'dart_utf_tag': '0.9.0+4',
+  'dart_watcher_tag': '0.9.7+7',
+  'dart_web_socket_channel_tag': 'corelib_2_2_1',
   'dart_yaml_tag': '2.1.13',
 
   # Build bot tooling for iOS
@@ -115,7 +115,7 @@ allowed_hosts = [
 ]
 
 deps = {
-  'src': 'https://github.com/flutter/buildroot.git' + '@' + 'e8854a250de400815604f2394352fa198b0b8c25',
+  'src': 'https://github.com/flutter/buildroot.git' + '@' + '9881cb0513c95c341e432670dbd1add0c76b7da9',
 
    # Fuchsia compatibility
    #