Check whether type parameters are loaded before emitting

test/codegen/expect/functions.js

+var functions = dart.defineLibrary(functions, {});
+var core = dart.import(core);
+(function(exports, core) {
+  'use strict';
+  function bootstrap() {
+    return dart.setType([new Foo()], core.List$(Foo));
+  }
+  dart.fn(bootstrap, () => dart.functionType(core.List$(Foo), []));
+  class Foo extends core.Object {}
+  dart.setSignature(Foo, {});
+  function main() {
+    core.print(bootstrap()[core.$get](0));
+  }
+  dart.fn(main, dart.void, []);
+  // Exports:
+  exports.bootstrap = bootstrap;
+  exports.Foo = Foo;
+  exports.main = main;
+})(functions, core);