export / import single class method using ES6 modules?

You have to export it as a separate variable/constant, like this:

class Foo {
  fooMethod() {};
}

export const fooMethod = Foo.prototype.fooMethod;

See Babel/repl

Edit

It turns out in comments that you don't really need an instance method (You don't use this). I would just define and use a regular function:

export function fooMethod(x) {
    return x + 1;
}

You would have to export it on the prototype. But remember that if you do that you won't call the function in the class/object context:

export foo.prototype. fooMethod

However I would recommend you to not to do so.

---

Okay, due to your comment you want a good way to have a common functionality for two classes, that don't extend the same base class. One simple way is to import a utility function from two classes:

foo.js

export function foo() {
  return this.name;
}

a.js

import {foo} from 'foo';
export class A extends BaseA {
  foo() {
    foo.apply(this, arguments);
  }
}

b.js

import {foo} from 'foo';
export class B extends BaseB {
  foo() {
    foo.apply(this, arguments);
  }
}

This is a good pattern and works well for a single function, but has limits if you want to apply more complex functionality. A good way to achieve this is a mixing pattern:

foo.js

export default superClass => class extends superClass {
  foo() {
    return this.name;
  }
};

a.js

import foo from 'foo';
export class A extends foo(BaseA) {
  ..
}

b.js

import foo from 'foo';
export class B extends foo(BaseB) {
  ..
}

This will make your mixing create a new anonymous class between your class 'A'/'B' and 'BaseA'/'BaseB', which provides the common function foo.