ComputeEngineConfig should implement TokenFetcher.

google/google.go

@@ -146,3 +146,10 @@ func (c *ComputeEngineConfig) FetchToken(existing *oauth2.Token) (token *oauth2.
 	}
 	return
 }
+
+// Cache returns nil. On Google Compute Engine, access tokens are
+// retrieved from the metaserver, no other persistence layer is
+// required.
+func (c *ComputeEngineConfig) Cache() oauth2.Cache {
+	return nil
+}