Correct update expression Number coercion (#7766)

* Correct update expression Number coercion

You have to `ToNumber` whatever the `UpdateExpression` argument is.

* Fix systemjs update expression

packages/babel-plugin-transform-modules-commonjs/test/fixtures/update-expression/negative-suffix/output.js

@@ -9,7 +9,7 @@ let diffLevel = 0;
 exports.diffLevel = diffLevel;
 
 function diff() {
-  if (!(exports.diffLevel = diffLevel = diffLevel - 1)) {
+  if (!(exports.diffLevel = diffLevel = +diffLevel - 1)) {
     console.log("hey");
   }
 }