Validation for exposures of types in attributes/operations

[saschanaz]

For example, this is invalid:

[Exposed=Window]
interface Foo {};

[Exposed=(Window, Worker)] 
interface Bar {
  attribute Foo foo; // Foo is not available on workers!
};

[saschanaz]

See also w3c/hr-time#130, which basically caused:

[Exposed=(Window,Worker)]
interface PermormanceMark { /* ... */ };

[Exposed=*]
interface Performance { 
  /* ... */
  PerformanceMark mark(/* ... */);
};