Fix more lint style issues in runtime patches.

runtime/lib/symbol_patch.dart

 // Copyright (c) 2013, 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.
 
+import 'dart:core' hide Symbol;
+
 @patch class Symbol {
   @patch const Symbol(String name)
       : this._name = name;
 
   @patch toString() => 'Symbol("${getUnmangledName(this)}")';
 
   static getUnmangledName(Symbol symbol) {
     String string = Symbol.getName(symbol);
 
     // Remove closurization hash mark
@@ skipping 35 matching lines @@
       result.write('=');
     }
     return result.toString();
   }
 
   @patch int get hashCode {
     const arbitraryPrime = 664597;
     return 0x1fffffff & (arbitraryPrime * _name.hashCode);
   }
 }