fix #470, string interpolation now respects Dart toString

lib/src/compiler/code_generator.dart

 // Copyright (c) 2015, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 import 'dart:collection' show HashMap, HashSet;
 import 'dart:math' show min, max;
 
 import 'package:analyzer/analyzer.dart' hide ConstantEvaluator;
 import 'package:analyzer/dart/ast/ast.dart';
 import 'package:analyzer/dart/ast/token.dart' show Token, TokenType;
@@ skipping 4170 matching lines @@
 
   @override
   JS.LiteralString visitSimpleStringLiteral(SimpleStringLiteral node) =>
       js.escapedString(node.value, node.isSingleQuoted ? "'" : '"');
 
   @override
   JS.Expression visitAdjacentStrings(AdjacentStrings node) =>
       _visitListToBinary(node.strings, '+');
 
   @override
-  JS.TemplateString visitStringInterpolation(StringInterpolation node) {
-    // Assuming we implement toString() on our objects, we can avoid calling it
-    // in most cases. Builtin types may differ though. We could handle this with
-    // a tagged template.
-    return new JS.TemplateString(_visitList(node.elements));
+  JS.Expression visitStringInterpolation(StringInterpolation node) {
+    return new JS.TaggedTemplate(
+        js.call('dart.str'), new JS.TemplateString(_visitList(node.elements)));
   }
 
   @override
   String visitInterpolationString(InterpolationString node) {
     // TODO(jmesserly): this call adds quotes, and then we strip them off.
     var str = js.escapedString(node.value, '`').value;
     return str.substring(1, str.length - 1);
   }
 
   @override
@@ skipping 288 matching lines @@
 }
 
 bool isLibraryPrefix(Expression node) =>
     node is SimpleIdentifier && node.staticElement is PrefixElement;
 
 LibraryElement _getLibrary(AnalysisContext c, String uri) =>
     c.computeLibraryElement(c.sourceFactory.forUri(uri));
 
 bool _isDartRuntime(LibraryElement l) =>
     l.isInSdk && l.source.uri.toString() == 'dart:_runtime';