fix #470, string interpolation now respects Dart toString

test/codegen/expect/DeltaBlue.js

 dart_library.library('DeltaBlue', 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 DeltaBlue$ = Object.create(null);
   const BenchmarkBase$ = Object.create(null);
   DeltaBlue$.main = function() {
@@ skipping 34 matching lines @@
         this.warmup();
       }), 100);
       let result = BenchmarkBase$.BenchmarkBase.measureFor(dart.fn(() => {
         this.exercise();
       }), 2000);
       this.teardown();
       return result;
     }
     report() {
       let score = this.measure();
-      core.print(`${this.name}(RunTime): ${score} us.`);
+      core.print(dart.str`${this.name}(RunTime): ${score} us.`);
     }
   };
   dart.setSignature(BenchmarkBase$.BenchmarkBase, {
     constructors: () => ({new: [BenchmarkBase$.BenchmarkBase, [core.String]]}),
     methods: () => ({
       run: [dart.void, []],
       warmup: [dart.void, []],
       exercise: [dart.void, []],
       setup: [dart.void, []],
       teardown: [dart.void, []],
@@ skipping 459 matching lines @@
       prev = v;
     }
     new DeltaBlue$.StayConstraint(last, DeltaBlue$.STRONG_DEFAULT);
     let edit = new DeltaBlue$.EditConstraint(first, DeltaBlue$.PREFERRED);
     let plan = DeltaBlue$.planner.extractPlanFromConstraints(dart.list([edit], DeltaBlue$.Constraint));
     for (let i = 0; i < 100; i++) {
       first.value = i;
       plan.execute();
       if (last.value != i) {
         core.print("Chain test failed:");
-        core.print(`Expected last value to be ${i} but it was ${last.value}.`);
+        core.print(dart.str`Expected last value to be ${i} but it was ${last.value}.`);
       }
     }
   };
   dart.fn(DeltaBlue$.chainTest, dart.void, [core.int]);
   DeltaBlue$.projectionTest = function(n) {
     DeltaBlue$.planner = new DeltaBlue$.Planner();
     let scale = new DeltaBlue$.Variable("scale", 10);
     let offset = new DeltaBlue$.Variable("offset", 1000);
     let src = null, dst = null;
     let dests = dart.list([], DeltaBlue$.Variable);
@@ skipping 25 matching lines @@
       v.value = newValue;
       plan.execute();
     }
     edit.destroyConstraint();
   };
   dart.fn(DeltaBlue$.change, dart.void, [DeltaBlue$.Variable, core.int]);
   DeltaBlue$.planner = null;
   BenchmarkBase$.Expect = class Expect extends core.Object {
     static equals(expected, actual) {
       if (!dart.equals(expected, actual)) {
-        dart.throw(`Values not equal: ${expected} vs ${actual}`);
+        dart.throw(dart.str`Values not equal: ${expected} vs ${actual}`);
       }
     }
     static listEquals(expected, actual) {
       if (expected[dartx.length] != actual[dartx.length]) {
-        dart.throw(`Lists have different lengths: ${expected[dartx.length]} vs ${actual[dartx.length]}`);
+        dart.throw(dart.str`Lists have different lengths: ${expected[dartx.length]} vs ${actual[dartx.length]}`);
       }
       for (let i = 0; i < dart.notNull(actual[dartx.length]); i++) {
         BenchmarkBase$.Expect.equals(expected[dartx.get](i), actual[dartx.get](i));
       }
     }
     fail(message) {
       dart.throw(message);
     }
   };
   dart.setSignature(BenchmarkBase$.Expect, {
     methods: () => ({fail: [dart.dynamic, [dart.dynamic]]}),
     statics: () => ({
       equals: [dart.void, [dart.dynamic, dart.dynamic]],
       listEquals: [dart.void, [core.List, core.List]]
     }),
     names: ['equals', 'listEquals']
   });
   // Exports:
   exports.DeltaBlue = DeltaBlue$;
   exports.BenchmarkBase = BenchmarkBase$;
 });