Add StackTrace.current getter.

tests/corelib/stacktrace_current_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 "dart:convert" show LineSplitter;
+
+import "package:expect/expect.dart";
+
+void main() {
+  var st0;
+  var st1;
+  // Primitive way to get stack trace,.
+  try { throw 0; } catch (_, s) { st0 = s; }
+  st1 = StackTrace.current;
+
+  var st0s = findMain(st0);
+  var st1s = findMain(st1);
+  // Stack traces are not equal (contains at least a different line number,
+  // and possible different frame numbers).
+  // They are *similar*, so check that they agree on everything but numbers.
+  var digits = new RegExp(r"\d+");
+  Expect.equals(st0s.replaceAll(digits, "0"), st1s.replaceAll(digits, "0"));
+}
+
+String findMain(StackTrace stack) {
+  var string = "$stack";
+  var lines = LineSplitter.split(string).toList();
+  while (lines.isNotEmpty && !lines.first.contains("main")) {
+    lines.removeAt(0);
+  }
+  return lines.join("\n");
+}