[react-packager] Move async dependencies to System.import

Summary:
We've decided to move the syntax for asynchronously requiring async dependencies. The new syntax works better with promises and therefore withe async/await as well. The new syntax looks like this: `System.import('moduleA').then(moduleA => {...});` or if you're using async/await you could simply do:
  let moduleA = await System.import('moduleA');
  new moduleA().someFunction();

If you need to require multiple dependencies just do:
  Promise
    .all([System.import('moduleA'), System.import('moduleB')])
    .then((moduleA, moduleB) => {...})

or the equivalent using async/await

packager/react-packager/src/DependencyResolver/replacePatterns.js

@@ -11,4 +11,5 @@
 
 exports.IMPORT_RE = /(\bimport\s+?(?:.+\s+?from\s+?)?)(['"])([^'"]+)(\2)/g;
 exports.REQUIRE_RE = /(\brequire\s*?\(\s*?)(['"])([^'"]+)(\2\s*?\))/g;
-exports.REQUIRE_ENSURE_RE = /\brequire\.ensure\s*\(\s*(?:\[([^\]]+)\])?/g;
+exports.SYSTEM_IMPORT_RE = /(\bSystem\.import\s*?\(\s*?)(['"])([^'"]+)(\2\s*?\))/g;
+