Add StackTrace.fromString constructor.

tests/corelib/stacktrace_fromstring_test.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.
+
+import "package:expect/expect.dart";
+import "package:async_helper/async_helper.dart";
+import "dart:async";
+
+void main() {
+  StackTrace stack;
+  try { throw 0; } catch (e, s) { stack = s; }
+  var string = "$stack";
+  StackTrace stringTrace = new StackTrace.fromString(string);
+  Expect.isTrue(stringTrace is StackTrace);
+  Expect.equals(stack.toString(), stringTrace.toString());
+
+  string = "some random string, nothing like a StackTrace";
+  stringTrace = new StackTrace.fromString(string);
+  Expect.isTrue(stringTrace is StackTrace);
+  Expect.equals(string, stringTrace.toString());
+
+  // Use stacktrace asynchronously.
+  asyncStart();
+  var c = new Completer();
+  c.completeError(0, stringTrace);
+  c.future.then((v) {
+    throw "Unexpected value: $v";
+  }, onError: (e, s) {
+    Expect.equals(string, s.toString());
+  }).then((_) {
+    var c = new StreamController();
+    c.stream.listen((v) {
+      throw "Unexpected value: $v";
+    }, onError: (e, s) {
+      Expect.equals(string, s.toString());
+      asyncEnd();
+    });
+    c.addError(0, stringTrace);
+    c.close();
+  });
+}