fix #577 - correctly toString ES Symbols

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 441 matching lines @@
   }
 
   var extension = getExtensionType(obj);
   if (extension != null) {
     return JS('', '#[dartx.hashCode]', obj);
   }
   return JS('', '#.hashCode', obj);
 }
 
 @JSExportName('toString')
-_toString(obj) {
+String _toString(obj) {
   if (obj == null) return "null";
 
   var extension = getExtensionType(obj);
   if (extension != null) {
-    return JS('', '#[dartx.toString]()', obj);
+    return JS('String', '#[dartx.toString]()', obj);
   }
-  return JS('', '"" + #', obj);
+  // TODO(jmesserly): restore this faster path once ES Symbol is treated as
+  // an extension type (and thus hits the above code path).
+  // See https://github.com/dart-lang/dev_compiler/issues/578.
+  // return JS('', '"" + #', obj);
+  return JS('String', '#.toString()', obj);
 }
 
 // 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 36 matching lines @@
     constructor(dartIterator) {
       this.dartIterator = dartIterator;
     }
     next() {
       let i = this.dartIterator;
       let done = !i.moveNext();
       return { done: done, value: done ? void 0 : i.current };
     }
   }
 ''');