fix #470, string interpolation now respects Dart toString

test/codegen/expect/misc.js

 dart_library.library('misc', 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 misc = Object.create(null);
   misc._Uninitialized = class _Uninitialized extends core.Object {
     new() {
@@ skipping 33 matching lines @@
     }
     ['=='](obj) {
       return dart.is(obj, misc.Derived) && obj.z == this.z && super['=='](obj);
     }
   };
   misc._isWhitespace = function(ch) {
     return ch == ' ' || ch == '\n' || ch == '\r' || ch == '\t';
   };
   dart.fn(misc._isWhitespace, core.bool, [core.String]);
   misc.expr = 'foo';
-  misc._escapeMap = dart.const(dart.map({'\n': '\\n', '\r': '\\r', '\f': '\\f', '\b': '\\b', '\t': '\\t', '\v': '\\v', '': '\\x7F', [`\${${misc.expr}}`]: ''}));
+  misc._escapeMap = dart.const(dart.map({'\n': '\\n', '\r': '\\r', '\f': '\\f', '\b': '\\b', '\t': '\\t', '\v': '\\v', '': '\\x7F', [dart.str`\${${misc.expr}}`]: ''}));
   misc.main = function() {
     core.print(dart.toString(1));
     core.print(dart.toString(1.0));
     core.print(dart.toString(1.1));
     let x = 42;
     core.print(dart.equals(x, dart.wrapType(dart.dynamic)));
     core.print(dart.equals(x, dart.wrapType(misc.Generic)));
     core.print(new (misc.Generic$(core.int))().type);
     core.print(dart.equals(new misc.Derived(), new misc.Derived()));
     new (misc.Generic$(core.int))().m();
   };
   dart.fn(misc.main);
   // Exports:
   exports.misc = misc;
 });