Stop relying on language version 2.0 in dartdoc tests. (#3601)

Dart no longer supports language versions prior to 2.12, so dartdoc
tests need to use a modern language version.

Changing the tests to use a language version of 3.0 helps unblock the
removal of legacy (pre-null-safety) support from the Dart analyzer.

Author: stereotype441

test/src/utils.dart

@@ -215,13 +215,13 @@ two:lib/
       "name": "one",
       "rootUri": "../../one",
       "packageUri": "lib/",
-      "languageVersion": "2.0"
+      "languageVersion": "3.0"
     },
     {
       "name": "two",
       "rootUri": "../",
       "packageUri": "lib/",
-      "languageVersion": "2.0"
+      "languageVersion": "3.0"
     }
   ],
   "generated": "2020-07-07T15:25:30.566271Z",