Duplicate names for constants

[brianquinlan]

ffigen generates many duplicate names for the same constants. For example:

const int noErr$3 = 0;
const int noErr$4 = 0;
const int noErr$7 = 0;
const int noErr$8 = 0;
const int noErr$2 = 0;
const int noErr$9 = 0;
const int noErr$6 = 0;
const int noErr$1 = 0;
const int noErr$10 = 0;
const int noErr$5 = 0;
const int noErr = 0;

See dart-lang/http#1850

[liamappelbe]

_addUnNamedEnumConstant adds enum constants to the bindings index, but never checks if the binding is already in there. They should be deduped by USR. A quick fix is just to add the deduping logic, but the better fix is to rewrite the bindings index system so this doesn't happen again. Macros also seem to have the same issue.