Add variance node type and generate property variance annotations (#4697)

* Add variance node type and generate property variance annotations

babel/babylon#161 adds parsing support for property variance
annotations. This PR adds the necessary node type for the new Variance
node and generate support for all the positions where variance can now
appear.

* Variance is no longer a separate node type

This diff also adds tests to class properties and to the
flow-strip-types transform.

* Add test + fix for edge case with variance and class proeprties

packages/babel-generator/src/generators/classes.js

@@ -60,6 +60,7 @@ export function ClassProperty(node: Object) {
     this.print(node.key, node);
     this.token("]");
   } else {
+    this._variance(node);
     this.print(node.key, node);
   }
   this.print(node.typeAnnotation, node);