Class methods: generate named functions (for debugging)

For class methods, generate named functions to make debugging easier.

Before:
```
class Foo {
  bar() {}
  static baz() {}
}

// ->

($traceurRuntime.createClass)(Foo, {
  bar: function() {}
}, {
  baz: function() {}
});
```

After:
```
class Foo {
  bar() {}
  static baz() {}
}

// ->

($traceurRuntime.createClass)(Foo, {
  bar: function Foo_bar() {}
}, {
  baz: function Foo$baz() {}
});
```