Add ArrowFunctionExpression to opaqueTypes

[yavorsky]

Using arrow functions inside generators:

function* gen () {
  const a = () => { 
    return 1 
  };
};

throws:

Error: src/index.js: unknown Expression of type "ArrowFunctionExpression"
    at /Users/aqson/Development/temp/babel-preset-env-issue-160/node_modules/regenerator-transform/lib/emit.js:1035:15

But works fine with FunctionExpressions and arrow functions without BlockStatements:

function gen* {
  const a = () => 1;
};

function gen2* {
  const a = function () {
    return 1;
  };
};

The root of the problems lies in the explodeStatement method where we check node for the leaps and 2 of working cases returns false, but the one with ArrowFunctionExpression + BlockStatement - true.

Resolves: babel/babel-preset-env#160 and other cases without babel-plugin-transform-es2015-arrow-functions.

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[benjamn]

This makes a ton of sense!