title: Compile solidity with solc-js but get empty result
body: <p>I try to compile the following contract (tested on <a href="https://remix.ethereum.org" rel="nofollow noreferrer">remix</a>) with <a href="https://github.com/ethereum/solc-js" rel="nofollow noreferrer">solc-js</a></p>
  
  <pre><code>pragma solidity ^0.4.21;
  
  contract Calculator {
      uint8 public result = 0;
  
      function add(uint8 value) public {
          result = result + value;
      }
  }
  </code></pre>
  
  <p>This is the code I am using</p>
  
  <pre><code>const { readFileSync } = require('fs');
  const solc = require('solc');
  const Web3 = require('web3');
  
  // connect to the local instance
  const web3 = new Web3(new Web3.providers.HttpProvider("http://localhost:8545"));
  
  const params = {
    language: "Solidity",
    sources: {
      'example': {
        content: readFileSync('./contracts/example.sol', 'utf-8')
      }
    }
  };
  
  const compiled = JSON.parse(solc.compileStandardWrapper(JSON.stringify(params)));
  
  // check the result
  console.log(compiled);
  </code></pre>
  
  <p>I follow the instruction from the <a href="https://github.com/ethereum/solc-js#from-version-0411" rel="nofollow noreferrer">solc repo</a> and the <a href="https://solidity.readthedocs.io/en/develop/using-the-compiler.html#input-description" rel="nofollow noreferrer">compiler input JSON spec</a>. It works without any error but the result is very empty</p>
  
  <pre><code>{
    "contracts": {
      "example": {
        "Calculator": {
          "evm": {}
        }
      }
    },
    "sources": {
      "example": {
        "id": 0
      }
    }
  }
  </code></pre>
  
  <p>It doesn't have a lot of content as described at the <a href="https://solidity.readthedocs.io/en/develop/using-the-compiler.html#output-description" rel="nofollow noreferrer">output description</a>. I am not sure what goes wrong in my code.</p>