1. Create the following three files.
2. Run gopherjs serve and load alltom/ in your browser.
3. Edit iface.go to change Bar() int to Bar2() int.
4. Edit main.go to change impl.MakeFoo().Bar() to impl.MakeFoo().Bar2().
5. Reload your browser.

Instead of seeing a compile error about FooImpl no longer implementing Foo, you get a runtime error saying "Uncaught TypeError: impl.MakeFoo(...).Bar2 is not a function". It looks like gopherjs serve doesn't realize that impl.go needs to be recompiled.

alltom/iface/iface.go

package iface

type Foo interface {
	Bar() int
}

alltom/impl/impl.go

package impl

import "alltom/iface"

type FooImpl int

func MakeFoo() iface.Foo {
	return FooImpl(0)
}

func (f FooImpl) Bar() int {
	return 42
}

alltom/main.go

package main

import "alltom/impl"

func main() {
	impl.MakeFoo().Bar()
}