Move generated files to gen/.

test/codegen/expect/temps.js

-dart_library.library('temps', 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 temps = Object.create(null);
-  const _x = Symbol('_x');
-  const __x = Symbol('__x');
-  const _function = Symbol('_function');
-  temps.FormalCollision = class FormalCollision extends core.Object {
-    new(x, _x$, func) {
-      this[_x] = x;
-      this[__x] = _x$;
-      this[_function] = func;
-    }
-  };
-  dart.setSignature(temps.FormalCollision, {
-    constructors: () => ({new: [temps.FormalCollision, [core.int, core.int, core.Function]]})
-  });
-  const _opt = Symbol('_opt');
-  temps.OptionalArg = class OptionalArg extends core.Object {
-    new(opt) {
-      if (opt === void 0) opt = 123;
-      this[_opt] = opt;
-      this.opt = null;
-    }
-    named(opts) {
-      let opt = opts && 'opt' in opts ? opts.opt : 456;
-      this.opt = opt;
-      this[_opt] = null;
-    }
-  };
-  dart.defineNamedConstructor(temps.OptionalArg, 'named');
-  dart.setSignature(temps.OptionalArg, {
-    constructors: () => ({
-      new: [temps.OptionalArg, [], [core.int]],
-      named: [temps.OptionalArg, [], {opt: core.int}]
-    })
-  });
-  temps.main = function() {
-    core.print(new temps.FormalCollision(1, 2, dart.fn(x => x)));
-    core.print(new temps.OptionalArg()[_opt]);
-    core.print(new temps.OptionalArg.named()[_opt]);
-  };
-  dart.fn(temps.main);
-  // Exports:
-  exports.temps = temps;
-});