First step of isolating a bunch of code that is used for generating
code, both within a GWT generator or as a separate build step for
building server-side implementations.

Also, this improves StringGenerator to produce String-valued
expressions like those before SafeHtml was added.

Public review at http://gwt-code-reviews.appspot.com/1422807


git-svn-id: https://google-web-toolkit.googlecode.com/svn/trunk@10060 8db76d5a-ed1c-0410-87a9-c151d255dfc7