7428 Download dynamic patch to separate file to avoid races (cla: yes)

7429 Eliminate unused import of ZipException (cla: yes)

7431 Dart SDK roll for 2019-01-09 (cla: yes)

7432 Improve 404 handling when downloading dynamic patches. (cla: yes)

7433 Eliminate std::string using directive (cla: yes)

7435 Add Ahem to LibTxt testing fonts. (cla: yes)

7436 Clear the font collection’s cache when a font is dynamically loaded (cla: yes)

7437 Temporary revert of Dart SDK rolls made since 2019/01/08 (cla: yes)

7439 Remove legacy and deprecated defaultClipBehavior (cla: yes)