Implement interface should not add optional properties

From https://github.com/Microsoft/vscode/issues/46286

TypeScript Version: 2.8.0-dev.20180320

Search Terms:

• implement interface
• quick fix
• code action
• optional property

Code

interface IFoo {
    x?: number
    y: number
}

class Foo implements IFoo {

}

Trigger implement interface on Foo

Expected behavior:
Only required properties required:

interface IFoo {
    x?: number
    y: number
}

class Foo implements IFoo {
    y: number;
}

Actual behavior:
Both required and optional properties added:

interface IFoo {
    x?: number
    y: number
}

class Foo implements IFoo {
    x?: number;
    y: number;
}

Start at the TypeScript language-service code action invoked by “implement interface” and reproduce the provided IFoo/Foo example. Check the generated implementation members, then verify that only the required y property is added while optional x is omitted.