fix #470, string interpolation now respects Dart toString

tool/input_sdk/private/ddc_runtime/operations.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.
 
 /// This library defines runtime operations on objects used by the code
 /// generator.
 part of dart._runtime;
 
 _canonicalFieldName(obj, name, args, displayName) => JS('', '''(() => {
   $name = $canonicalMember($obj, $name);
@@ skipping 448 matching lines @@
 }
 
 @JSExportName('toString')
 _toString(obj) {
   if (obj == null) return "null";
 
   var extension = getExtensionType(obj);
   if (extension != null) {
     return JS('', '#[dartx.toString]()', obj);
   }
   return JS('', '#.toString()', obj);

[sra1, 2016/05/17 23:47:58]

It is faster to do:

JS('', '"" + #', obj);

JS engines optimize + expressions with strings.

[Jennifer Messerly, 2016/05/18 00:23:42]

Done.

 }
 
 // TODO(jmesserly): is the argument type verified statically?
 noSuchMethod(obj, Invocation invocation) {
   if (obj == null) {
     throw new NoSuchMethodError(
         null,
         invocation.memberName,
         invocation.positionalArguments,
         invocation.namedArguments);
@@ skipping 12 matching lines @@
   if (result != null) return wrapType(result);
 
   // Delegate to the (possibly user-defined) method on the object.
   var extension = getExtensionType(obj);
   if (extension != null) {
     return JS('', '#[dartx.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) {
       this.dartIterator = dartIterator;
     }
     next() {
       let i = this.dartIterator;
       let done = !i.moveNext();
       return { done: done, value: done ? void 0 : i.current };
     }
   }
 ''');