Assertion error when a class extends and implements

Trying to compile
```
class B {
}

interface I {
}

export class A extends B implements I {
}
```
yields `Error [AssertionError]: assertion failed`

which comes from https://github.com/AssemblyScript/assemblyscript/blob/af3b48985d8eb4c1ff8cbff8ad22776fea8299bc/src/program.ts#L1220

Is this intended? Looks like it should be supported.