Add wrapPluginVisitorMethod option to allow introspection and metrics tracking of plugins

[sebmck]

I'll submit a PR to babel.github.io with docs once this gets accepted. This adds an optional callback that you can use to wrap visitor methods. At FB we're having problems with plugin performance and Babel 6 merging plugin visitors means we have little insight into performance, this will allow us to have granular tracking and telemetry. Usage is displayed in the included test:

  test("option wrapPluginVisitorMethod", function () {
    var calledRaw = 0;
    var calledIntercept = 0;

    babel.transform("function foo() { bar(foobar); }", {
      wrapPluginVisitorMethod(pluginAlias, visitorType, callback) {
        if (pluginAlias !== "foobar") {
          return callback;
        }

        assert.equal(visitorType, "enter");

        return function () {
          calledIntercept++;
          return callback.apply(this, arguments);
        };
      },

      plugins: [new Plugin({
        name: "foobar",
        visitor: {
          "Program|Identifier": function () {
            calledRaw++;
          }
        }
      })]
    });

    assert.equal(calledRaw, 4);
    assert.equal(calledIntercept, 4);
  });

[codecov-io]

Current coverage is 89.73% (diff: 100%)

Merging #3659 into master will increase coverage by 1.51%

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  Lines         13591      17454   +3863   
  Methods        1424       2080    +656   
  Messages          0          0           
  Branches       3127       4732   +1605   
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+ Hits          11990      15662   +3672   
- Misses         1601       1792    +191   
  Partials          0          0           

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[hzoo]

Are we able to add a name to the existing plugins (since I see you added them for the 2 internal ones)?

[sebmck]

@hzoo We actually already populate an internal name field if the plugin was created via a string reference.

[hzoo]

Cool I was wondering about that 😛

[hzoo]

👍

and @loganfsmyth said earlier

I'd love to add a note that it's not really meant for implementing anything, just inspection, when we do the docs