title: ES2015 vs ESM2015
body: <p>I want to understand the difference between ES2015 and ESM2015. I know that ES2015 is the ES^ standards but recently while compiling my Angular application using <code>ng build</code> I got the following : 
  <code>Compiling @angular/core : es2015 as esm2015</code></p>
  
  <p>I want to know why ESM2015? Is it minified ES2015 files?</p>