This adds an optional argument to generated getters for string fields to
specify the encoding type and overloads the createString() function. It's
now possible to use either JavaScript UTF-16 string objects or C-style
UTF-8 byte arrays (Uint8Array) for string data.
This adds a JavaScript language target. The generated JavaScript uses Google
Closure Compiler type annotations and can be compiled using the advanced
compilation mode, which performs type checking and optimizations such as
inlining and dead code elimination. The generated JavaScript also exports all
generated symbols for use with Node.js and RequireJS. This export behavior
can be turned off with the --no-js-exports flag for use with Google Closure
Compiler.