Declarations and ThrowStatement in an IfStatement within DoExpression breaks compilation

[phpnode]

Input Code

let x = 100;

let a = do {
  if(x > 10) {
    true;
  } else {
    let a = 123; // does not compile
    throw "nope"; // does not compile
  }
};

Example in the REPL

Babel Configuration (.babelrc, package.json, cli command)

Using es2015 and stage-0

Expected Behavior

It should wrap the body in an IIFE.

Current Behavior

It doesn't think that an IIFE is required but when the plugin tries to get the completion record for the relevant if statement it finds undefined and passes that to either the consequent or alternate of a ConditionalExpression, which throws:

Property alternate of ConditionalExpression expected node to be of a type ["Expression"] but instead got undefined

Possible Solution

Detect this scenario and wrap in an IIFE, or add a helper that turns throw e into _throw(e) and hoist + rename declarations.

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[jridgewell]

Fixed by #5693.