The compiler gives an error on the last line:

function s(p: {[k:string]:string}) {
    console.log(p);
}

let t: {[k:string]:string} = {thing: 'otherthing'};
s(t);

let t2 = {thing: 'otherthing'}; 
s(t2);

Index signature is missing in type '{thing: 'otherthing'}'

Which is very sad since I'm refactoring all of angular to use string index signatures instead of our hacky StringMap (angular/angular#4483)