Mangling function/class declaration names is dangerous

[Rich-Harris]

Code might refer to fn.name. Uglify, Closure and Babili all mangle function names by default, so maybe we should too. Either way we should certainly have the option to preserve function and class names.

(In an ideal world we might be able to determine whether it's safe to mangle a given function's name — e.g. in the following snippet there is no possibility of the name mattering, and we could determine as much statically):

function foo () {
  function bar () {
    console.log('the name of this function does not matter');
  }

  bar();
}

[Rich-Harris]

Incidentally, disabling mangling for classes and functions is relatively easy — just need to change this function:

Object.keys( this.declarations ).forEach( name => {
  const declaration = this.declarations[ name ];

+  if ( /^(Function|Class)/.test( declaration.node.parent.type ) && declaration.node === declaration.node.parent.id ) {
+    return;
+  }

  declaration.alias = this.createIdentifier( used );

  declaration.instances.forEach( instance => {
    const replacement = instance.parent.type === 'Property' && instance.parent.shorthand ?
      `${instance.name}:${declaration.alias}` :
      declaration.alias;

    code.overwrite( instance.start, instance.end, replacement, true );
  });
});

[Rich-Harris]

On the flip side, when mangling is enabled, we should remove function names for all FunctionExpression nodes whose names are not referenced inside the function (which should be easy to determine)