title: Are Flow.js maps not writable?
body: <pre><code>/* @flow */
  
  let m: Map&lt;number, string&gt;;
  
  m = new Map();
  
  m[1] = 'qwe';
  </code></pre>
  
  <p>gives this errors:</p>
  
  <pre><code>7: m[1] = 'qwe';
     ^ Cannot assign `'qwe'` to `m[1]` because an index signature declaring the expected key / value type is not writable.
  7: m[1] = 'qwe';
       ^ Cannot assign `'qwe'` to `m[1]` because number [1] is incompatible with `$SymbolToStringTag` [2].
  References:
  7: m[1] = 'qwe';
       ^ [1]
  [LIB] ..//static/v0.111.0/flowlib/core.js:606:     +[key: $SymbolToStringTag]: any;
                                                            ^ [2]
  </code></pre>
  
  <p>you can try it here
  <a href="https://flow.org/try/#0PQKgBAAgZgNg9gdzCYAoVMCmAXMBbALjAFkBDABwB4A7AVzwCNMAnAGjAGdtmBLagcwB8AbnR4wAXjDVMSMuQAUASlGo8AbQCMAXUlgA5AEcEmfcKA" rel="nofollow noreferrer">https://flow.org/try/#0PQKgBAAgZgNg9gdzCYAoVMCmAXMBbALjAFkBDABwB4A7AVzwCNMAnAGjAGdtmBLagcwB8AbnR4wAXjDVMSMuQAUASlGo8AbQCMAXUlgA5AEcEmfcKA</a></p>