[explicit-function-return-type] allowExpressions should not allow methods

[whatisaphone]

Repro

{
  "rules": {
    "@typescript-eslint/explicit-function-return-type": ["error", {"allowExpressions": true}]
  }
}

class C {
  public method = () => {};
}
const variable = () => {};

Expected Result

Both lines should get hit with the error: "Missing return type on function".

Actual Result

Only variable gets the error; method does not. method is erroneously allowed due to allowExpressions.

Additional Info

From reading old issues, it looks like the idea behind allowExpressions is to allow expressions, but not to allow pure declarations which consist of only a function expression (source). This categorization is working great already for variable declarartions. I think the allowExpressions option should follow the same reasoning for both, and also treat a class property method declaration as a function declaration.

Versions

package	version
@typescript-eslint/eslint-plugin	2.1.0
@typescript-eslint/parser	2.1.0
TypeScript	3.5.1
ESLint	6.3.0
node	8.10.0
npm	5.6.0