remix-project/libs/remix-simulator/src/genesis.ts

import EthJSBlock from 'ethereumjs-block'
import { BN } from 'ethereumjs-util'

export function generateBlock (executionContext) {
  const block: EthJSBlock = new EthJSBlock({
    header: {
      timestamp: (new Date().getTime() / 1000 | 0),
      number: 0,
      coinbase: '0x0e9281e9c6a0808672eaba6bd1220e144c9bb07a',
      difficulty: (new BN('69762765929000', 10)),
      gasLimit: new BN('8000000').imuln(1)
    },
    transactions: [],
    uncleHeaders: []
  })

  executionContext.vm().runBlock({ block: block, generate: true, skipBlockValidation: true, skipBalance: false }).then(() => {
    executionContext.addBlock(block)
  })
}