fix #470, string interpolation now respects Dart toString

test/codegen/expect/closure.js

 export const closure = Object.create(null);
 import { core, js, dart, dartx } from 'dart_sdk';
 closure.generic_function = function(T) {
   return (items: core.List<T>, seed: T): core.List<T> => {
-    let strings = items[dartx.map](core.String)(dart.fn((i: T): string => `${i}`, core.String, [T]))[dartx.toList]();
+    let strings = items[dartx.map](core.String)(dart.fn((i: T): string => dart.toString(i), core.String, [T]))[dartx.toList]();
     return items;
   };
 };
 dart.fn(closure.generic_function, T => [core.List$(T), [core.List$(T), T]]);
 closure.Callback = dart.typedef('Callback', () => dart.functionType(dart.void, [], {i: core.int}));
 closure.Foo$ = dart.generic(T => {
   class Foo<T> extends core.Object {
     i: number;
     b: boolean;
     s: string;
@@ skipping 71 matching lines @@
 });
 closure.main = function(args): void {
 };
 dart.fn(closure.main, dart.void, [dart.dynamic]);
 /** @final {string} */
 closure.some_top_level_constant = "abc";
 /** @final {string} */
 closure.some_top_level_final = "abc";
 /** @type {string} */
 closure.some_top_level_var = "abc";