Dev compiler debugger related tweaks.

pkg/dev_compiler/tool/input_sdk/private/ddc_runtime/runtime.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.
 
 library dart._runtime;
 
 import 'dart:async';
 import 'dart:collection';
 
+import 'dart:_debugger' show stackTraceMapper;
 import 'dart:_foreign_helper' show JS, JSExportName, rest, spread;
 import 'dart:_interceptors' show JSArray;
 import 'dart:_js_helper'
     show
         AssertionErrorWithMessage,
         BooleanConversionAssertionError,
         CastErrorImplementation,
         getTraceFromException,
         Primitives,
         TypeErrorImplementation,
@@ skipping 68 matching lines @@
       }
     }
     var globalState = (typeof window != "undefined") ? window
       : (typeof global != "undefined") ? global
       : (typeof self != "undefined") ? self : {};
     return globalState;
   }()
 ''');
 
 final JsSymbol = JS('', 'Symbol');