Add StackTrace.fromString constructor.

sdk/lib/core/stacktrace.dart

Index: sdk/lib/core/stacktrace.dart
diff --git a/sdk/lib/core/stacktrace.dart b/sdk/lib/core/stacktrace.dart
index ef8cc4dbb4adfac17f33fef5f28d31fe15d45a2c..c8540dc47bb840aa73fd256b5afd939426a2c440 100644
--- a/sdk/lib/core/stacktrace.dart
+++ b/sdk/lib/core/stacktrace.dart
@@ -14,6 +14,22 @@ part of dart.core;
  * 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.
    *
@@ -25,3 +41,8 @@ abstract class StackTrace {
   String toString();
 }
 
+class _StringStackTrace implements StackTrace {
+  final String _stackTrace;
+  _StringStackTrace(this._stackTrace);
+  String toString() => _stackTrace;
+}