fix #470, string interpolation now respects Dart toString

tool/input_sdk/private/ddc_runtime/operations.dart

Index: tool/input_sdk/private/ddc_runtime/operations.dart
diff --git a/tool/input_sdk/private/ddc_runtime/operations.dart b/tool/input_sdk/private/ddc_runtime/operations.dart
index c6d58bbdd172e7ad344880fd2ec34e55f5694403..e5de57819ebfbf26a955a6cf72c86788c9811169 100644
--- a/tool/input_sdk/private/ddc_runtime/operations.dart
+++ b/tool/input_sdk/private/ddc_runtime/operations.dart
@@ -499,6 +499,18 @@ runtimeType(obj) {
   return JS('', '#.runtimeType', obj);
 }
 
+/// Implements Dart's interpolated strings as ES2015 tagged template literals.
+///
+/// For example: dart.str`hello ${name}`
+String str(strings, @rest values) => JS('', '''(() => {
+  let s = $strings[0];
+  for (let i = 0, len = $values.length; i < len; ) {
+    s += $_toString($values[i]) + $strings[++i];
+  }
+  return s;
+})()''');
+
+
 final JsIterator = JS('', '''
   class JsIterator {
     constructor(dartIterator) {