Move generated files to gen/.

test/codegen/expect/covariance.js

-dart_library.library('covariance', null, /* Imports */[
-  'dart_sdk'
-], function(exports, dart_sdk) {
-  'use strict';
-  const core = dart_sdk.core;
-  const dart = dart_sdk.dart;
-  const dartx = dart_sdk.dartx;
-  const covariance = Object.create(null);
-  const _t = Symbol('_t');
-  covariance.Foo$ = dart.generic(T => {
-    class Foo extends core.Object {
-      new() {
-        this[_t] = null;
-      }
-      add(t) {
-        dart.as(t, T);
-        this[_t] = t;
-      }
-      forEach(fn) {
-        fn(this[_t]);
-      }
-    }
-    dart.setSignature(Foo, {
-      methods: () => ({
-        add: [dart.dynamic, [T]],
-        forEach: [dart.dynamic, [dart.functionType(dart.void, [T])]]
-      })
-    });
-    return Foo;
-  });
-  covariance.Foo = covariance.Foo$();
-  covariance.Bar = class Bar extends covariance.Foo$(core.int) {
-    new() {
-      super.new();
-    }
-    add(x) {
-      core.print(`Bar.add got ${x}`);
-      super.add(x);
-    }
-  };
-  dart.setSignature(covariance.Bar, {
-    methods: () => ({add: [dart.dynamic, [core.int]]})
-  });
-  covariance.main = function() {
-    let foo = new covariance.Bar();
-    foo.add('hi');
-  };
-  dart.fn(covariance.main);
-  // Exports:
-  exports.covariance = covariance;
-});