77 Errors with Flow 0.27.

[iamchenxin]

I find something strange.
Like uildASTSchema.js#L338:

  function makeImplementedInterfaces(def: ObjectTypeDefinition) {
    return def.interfaces.map(inter => produceTypeDef(inter));
  }

which def.interfaces is interfaces?: ?Array<NamedType> (defined in ast.js#L296 )
Seems flow 0.26 should give an error on those,but it shouldn't.

[iamchenxin]

Fixed these errors in PR #420 .