fix #470, string interpolation now respects Dart toString

test/codegen/expect/misc.js

Index: test/codegen/expect/misc.js
diff --git a/test/codegen/expect/misc.js b/test/codegen/expect/misc.js
index 68e5b743627035046ff083b76a9029bd9cc24622..d48f45eae1ec3f567f8b05913b173758ad72d98b 100644
--- a/test/codegen/expect/misc.js
+++ b/test/codegen/expect/misc.js
@@ -51,7 +51,7 @@ dart_library.library('misc', null, /* Imports */[
   };
   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));