Add StackTrace.fromString constructor.

sdk/lib/core/stacktrace.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.
 
 part of dart.core;
 
 /**
  * An interface implemented by all stack trace objects.
  *
  * A [StackTrace] is intended to convey information to the user about the call
  * sequence that triggered an exception.
  *
  * These objects are created by the runtime, it is not possible to create
  * them programmatically.
  */
 abstract class StackTrace {
+  StackTrace();  // In case existing classes extend StackTrace.
+
+  /**
+   * Create a `StackTrace` object from [stackTraceString].
+   *
+   * The created stack trace will have a `toString` method returning
+   * `stackTraceString`.
+   *
+   * The `stackTraceString` can be a string returned by some other
+   * stack trace, or it can be any string at all.
+   * If the string doesn't look like a stack trace, code that interprets
+   * stack traces is likely to fail, so fake stack traces should be used
+   * with care.
+   */
+  factory StackTrace.fromString(String stackTraceString) = _StringStackTrace;
+
   /**
    * Returns a [String] representation of the stack trace.
    *
    * The string represents the full stack trace starting from
    * the point where a throw ocurred to the top of the current call sequence.
    *
    * The exact format of the string representation is not final.
    */
   String toString();
 }
 
+class _StringStackTrace implements StackTrace {
+  final String _stackTrace;
+  _StringStackTrace(this._stackTrace);
+  String toString() => _stackTrace;
+}