fix #470, string interpolation now respects Dart toString

test/codegen/expect/notnull.js

 dart_library.library('notnull', 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 notnull = Object.create(null);
   notnull.intAssignments = function() {
     let i = 0;
@@ skipping 202 matching lines @@
     try {
       dart.throw(1);
     } catch (e) {
       core.print(dart.dsend(e, '+', 1));
     }
 
     try {
       dart.dsend(null, 'foo');
     } catch (e) {
       let trace = dart.stackTrace(e);
-      core.print(`${typeof e == 'string' ? e : dart.toString(e)} at ${trace}`);
+      core.print(dart.str`${typeof e == 'string' ? e : dart.toString(e)} at ${trace}`);
     }
 
   };
   dart.fn(notnull.controlFlow);
   notnull.cascadesOnNull = function() {
     let x = null;
     core.print(dart.hashCode(((() => {
       dart.toString(x);
       dart.runtimeType(x);
       return x;
@@ skipping 20 matching lines @@
     notnull.localEscapes();
     notnull.controlFlow();
     notnull.cascadesOnNull();
     notnull.nullableCycle();
     notnull.nonNullableCycle();
   };
   dart.fn(notnull.main);
   // Exports:
   exports.notnull = notnull;
 });