isDefineAMD fails when using arrow functions

[roguib]

Add the following test in test/tests.js. Run npm test and notice that fails:

assert.ok(check("define(['jquery'] , ($) => { class A {} return A;});", types.isDefineAMD));

Checkout version 2.7.0, which has commit hash of f5c2a2c. Execute the same test and notice that now it passes. This might be a regression of #18, although I'm not sure about it.