ES5 fails on class fields

[MichaelXF]

Describe the bug:

The es5 option errors when class fields are encountered

Config and Small code sample

Config:

{
  target: "node",
  es5: true,
}

Code:

class MyClass {
  myField = 1;
}

Error:

Node {
  type: 'PropertyDefinition',
  start: 23,
  end: 35,
  static: false,
  computed: false,
  key: Node { type: 'Identifier', start: 23, end: 30, name: 'myField' },
  value: Node { type: 'Literal', start: 33, end: 34, value: 1, raw: '1' }
}
(node:4288) UnhandledPromiseRejectionWarning: Error: Unsupported method definition