use js_ast instead of strings to generate JS

test/codegen/expect/cascade/cascade.js

 var cascade;
-(function (cascade) {
+(function(cascade) {
   'use strict';
   class A extends dart.Object {
     A() {
       this.x = null;
     }
   }
-
   // Function test_closure_with_mutate: () → void
   function test_closure_with_mutate() {
     let a = new A();
     a.x = () => {
       core.print("hi");
       a = null;
     };
     ((_) => {
-      dart.dinvoke(_, "x");
-      dart.dinvoke(_, "x");
+      dart.dinvoke(_, 'x');
+      dart.dinvoke(_, 'x');
     })(a);
     core.print(a);
   }
-
   // Function test_closure_without_mutate: () → void
   function test_closure_without_mutate() {
     let a = new A();
     a.x = () => {
       core.print(a);
     };
-    dart.dinvoke(a, "x");
-    dart.dinvoke(a, "x");
+    dart.dinvoke(a, 'x');
+    dart.dinvoke(a, 'x');
     core.print(a);
   }
-
   // Function test_mutate_inside_cascade: () → void
   function test_mutate_inside_cascade() {
     let a = null;
     a = ((_) => {
-      _.x = (a = null);
-      _.x = (a = null);
+      _.x = a = null;
+      _.x = a = null;
       return _;
     })(new A());
     core.print(a);
   }
-
   // Function test_mutate_outside_cascade: () → void
   function test_mutate_outside_cascade() {
     let a = null, b = null;
     a = new A();
-    dart.dput(a, "x", (b = null));
-    dart.dput(a, "x", (b = null));
+    dart.dput(a, 'x', b = null);
+    dart.dput(a, 'x', b = null);
     a = null;
     core.print(a);
   }
-
   // Function test_VariableDeclaration_single: () → void
   function test_VariableDeclaration_single() {
     let a = new List.from([]);
-    dart.dput(a, "length", 2);
+    dart.dput(a, 'length', 2);
     a.add(42);
     core.print(a);
   }
-
   // Function test_VariableDeclaration_last: () → void
   function test_VariableDeclaration_last() {
     let a = 42, b = new List.from([]);
-    dart.dput(b, "length", 2);
+    dart.dput(b, 'length', 2);
     b.add(a);
     core.print(b);
   }
-
   // Function test_VariableDeclaration_first: () → void
   function test_VariableDeclaration_first() {
     let a = ((_) => {
       _.length = 2;
       _.add(3);
       return _;
     })(new List.from([])), b = 2;
     core.print(a);
   }
-
   // Exports:
   cascade.A = A;
   cascade.test_closure_with_mutate = test_closure_with_mutate;
   cascade.test_closure_without_mutate = test_closure_without_mutate;
   cascade.test_mutate_inside_cascade = test_mutate_inside_cascade;
   cascade.test_mutate_outside_cascade = test_mutate_outside_cascade;
   cascade.test_VariableDeclaration_single = test_VariableDeclaration_single;
   cascade.test_VariableDeclaration_last = test_VariableDeclaration_last;
   cascade.test_VariableDeclaration_first = test_VariableDeclaration_first;
 })(cascade || (cascade = {}));