fix #470, string interpolation now respects Dart toString

test/codegen/expect/sunflower/sunflower.js

 dart_library.library('sunflower', null, /* Imports */[
   'dart_sdk'
 ], function(exports, dart_sdk) {
   'use strict';
   const core = dart_sdk.core;
   const html = dart_sdk.html;
   const math = dart_sdk.math;
   const dart = dart_sdk.dart;
   const dartx = dart_sdk.dartx;
   const sunflower = Object.create(null);
@@ skipping 42 matching lines @@
   sunflower.draw = function() {
     sunflower.seeds = core.int.parse(sunflower.slider[dartx.value]);
     sunflower.context[dartx.clearRect](0, 0, sunflower.MAX_D, sunflower.MAX_D);
     for (let i = 0; i < dart.notNull(sunflower.seeds); i++) {
       let theta = i * painter.TAU / dart.notNull(sunflower.PHI);
       let r = dart.notNull(math.sqrt(i)) * sunflower.SCALE_FACTOR;
       let x = sunflower.centerX + r * dart.notNull(math.cos(theta));
       let y = sunflower.centerY - r * dart.notNull(math.sin(theta));
       new sunflower.SunflowerSeed(x, y, sunflower.SEED_RADIUS).draw(sunflower.context);
     }
-    sunflower.notes[dartx.text] = `${sunflower.seeds} seeds`;
+    sunflower.notes[dartx.text] = dart.str`${sunflower.seeds} seeds`;
   };
   dart.fn(sunflower.draw, dart.void, []);
   circle.Circle = class Circle extends core.Object {
     new(x, y, radius) {
       this.x = x;
       this.y = y;
       this.radius = radius;
     }
   };
   dart.setSignature(circle.Circle, {
@@ skipping 46 matching lines @@
       return dart.as(painter.canvas[dartx.getContext]('2d'), html.CanvasRenderingContext2D);
     }
   });
   // Exports:
   exports.sunflower = sunflower;
   exports.circle = circle;
   exports.painter = painter;
 });
 
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