flatten: Fix undefined error

Without this change, running `grunt build` would generate this error:

    ...
    Transpiling...
    Minifying...
    Calculating main bundle(s)...
    Flattening main bundle(s)...
    ERROR: Cannot read property 'length' of undefined TypeError: Cannot read property 'length' of undefined
        at /Users/alvin/Projects/mathspace/www/msproblem/static/jsmvc/node_modules/steal-tools/lib/bundle/flatten.js:144:47
    ...

It seems that sometimes `node.minifiedSource` can be undefined, and that there's `node/source`  and `node/size` functions that can better handle this.