fix #470, string interpolation now respects Dart toString

test/codegen/expect/methods.js

 dart_library.library('methods', 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 methods = Object.create(null);
   const _c = Symbol('_c');
   methods.A = class A extends core.Object {
@@ skipping 48 matching lines @@
       z: [core.int, [], [core.num]],
       zz: [core.int, [], [core.int]],
       w: [core.int, [core.int], {b: core.num}],
       ww: [core.int, [core.int], {b: core.int}],
       clashWithObjectProperty: [dart.dynamic, [], {constructor: dart.dynamic}],
       clashWithJsReservedName: [dart.dynamic, [], {function: dart.dynamic}]
     })
   });
   methods.Bar = class Bar extends core.Object {
     call(x) {
-      return core.print(`hello from ${x}`);
+      return core.print(dart.str`hello from ${x}`);
     }
   };
   dart.setSignature(methods.Bar, {
     methods: () => ({call: [dart.dynamic, [dart.dynamic]]})
   });
   methods.Foo = class Foo extends core.Object {
     new() {
       this.bar = new methods.Bar();
     }
   };
   methods.test = function() {
     let f = new methods.Foo();
     dart.dsend(f, 'bar', "Bar's call method!");
     let a = new methods.A();
     let g = dart.bind(a, 'x');
     let aa = new methods.A();
     let h = dart.dload(aa, 'x');
     let ts = dart.bind(a, 'toString');
     let nsm = dart.bind(a, 'noSuchMethod');
     let c = dart.bind("", dartx.padLeft);
     let r = dart.bind(3.0, dartx.floor);
   };
   dart.fn(methods.test);
   // Exports:
   exports.methods = methods;
 });