use js_ast instead of strings to generate JS

test/codegen/expect/fieldtest/fieldtest.js

 var fieldtest;
-(function (fieldtest) {
+(function(fieldtest) {
   'use strict';
   class A extends dart.Object {
     A() {
       this.x = 42;
     }
   }
-
   let B$ = dart.generic(function(T) {
     class B extends dart.Object {
       B() {
         this.x = dart.as(null, core.int);
         this.y = null;
         this.z = dart.as(null, T);
       }
     }
     return B;
   });
   let B = B$(dynamic);
-
   // Function foo: (A) → int
   function foo(a) {
     core.print(a.x);
     return a.x;
   }
-
   // Function bar: (dynamic) → int
   function bar(a) {
-    core.print(dart.dload(a, "x"));
-    return dart.as(dart.dload(a, "x"), core.int);
+    core.print(dart.dload(a, 'x'));
+    return dart.as(dart.dload(a, 'x'), core.int);
   }
-
   // Function baz: (A) → dynamic
-  function baz(a) { return a.x; }
-
+  function baz(a) {
+    return a.x;
+  }
   // Function compute: () → int
-  function compute() { return 123; }
-
+  function compute() {
+    return 123;
+  }
   dart.defineLazyProperties(fieldtest, {
-    get y() { return compute() + 444 },
-    set y(x) {},
+    get y() {
+      return compute() + 444;
+    },
+    set y() {}
   });
-
   dart.copyProperties(fieldtest, {
-    get q() { return core.String['+'](core.String['+']('life, ', 'the universe '), 'and everything'); },
-    get z() { return 42; },
+    get q() {
+      return core.String['+'](core.String['+']('life, ', 'the universe '), 'and everything');
+    },
+    get z() {
+      return 42;
+    },
     set z(value) {
       fieldtest.y = dart.as(value, core.int);
-    },
+    }
   });
-
   // Function main: () → void
   function main() {
     let a = new A();
     foo(a);
     bar(a);
     core.print(baz(a));
   }
-
   // Exports:
   fieldtest.A = A;
   fieldtest.B = B;
   fieldtest.B$ = B$;
   fieldtest.foo = foo;
   fieldtest.bar = bar;
   fieldtest.baz = baz;
   fieldtest.compute = compute;
+  fieldtest.q = q;
+  fieldtest.z = z;
+  fieldtest.z = z;
   fieldtest.main = main;
 })(fieldtest || (fieldtest = {}));