-Spit string_test into string_test and string_static_test
-Added various MissingCompileTimeError entries in status files for
tests which throw exceptions on configurations that haven't implemented Dart 2.0
-Switched var -> dynamic in symbol_operator_test
BUG=
R=jcollins@google.com, rnystrom@google.com
Review-Url: https://codereview.chromium.org/2983253002 .
Ben Konyi