Automated g4 rollback of changelist 215970442.

*** Reason for rollback ***

Rolling forward

*** Original change description ***

Automated g4 rollback of changelist 215923974.

*** Reason for rollback ***

Broke some AWV tests: https://test.corp.google.com/ui#t=//ads/awapps2/video/campaign_construction:fakestack_campaign_settings_actions_webdriver_test_chrome-linux&cl=215923974&flags=CAMQAg==

*** Original change description ***

Flip default Dart version to 2.1.0-dev.6.0.

***

***

PiperOrigin-RevId: 216235100

_goldens/test/_files/dart2js/dart2js_golden.golden

@@ -1,4 +1,4 @@
-/* LICENSES listed at end of file */// Generated by dart2js (full emitter, strong, trust primitives, omit checks), the Dart to JavaScript compiler version: 2.1.0-dev.5.0.
+/* LICENSES listed at end of file */// Generated by dart2js (full emitter, strong, trust primitives, omit checks), the Dart to JavaScript compiler version: 2.1.0-dev.6.0.
 // The code supports the following hooks:
 // dartPrint(message):
 //    if this function is defined it is called instead of the Dart [print]