Refactor runtime into libraries, better type reps

test/codegen/expect/cascade.js

-dart.library('cascade', null, /* Imports */[
+dart_library.library('cascade', null, /* Imports */[
+  "dart_runtime/dart",
   'dart/core'
 ], /* Lazy imports */[
-], function(exports, core) {
+], function(exports, dart, core) {
   'use strict';
+  let dartx = dart.dartx;
   class A extends core.Object {
     A() {
       this.x = null;
     }
   }
   function test_closure_with_mutate() {
     let a = new A();
     a.x = dart.fn(() => {
       core.print("hi");
       a = null;
@@ skipping 98 matching lines @@
   exports.test_mutate_inside_cascade = test_mutate_inside_cascade;
   exports.test_mutate_outside_cascade = test_mutate_outside_cascade;
   exports.test_VariableDeclaration_single = test_VariableDeclaration_single;
   exports.test_VariableDeclaration_last = test_VariableDeclaration_last;
   exports.test_VariableDeclaration_first = test_VariableDeclaration_first;
   exports.test_increment = test_increment;
   exports.Base$ = Base$;
   exports.Base = Base;
   exports.Foo = Foo;
 });