Add StackTrace.current getter.

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 c8540dc47bb840aa73fd256b5afd939426a2c440..17abd3831ed957102d61eaa16093b9e31e33a8a4 100644
--- a/sdk/lib/core/stacktrace.dart
+++ b/sdk/lib/core/stacktrace.dart
@@ -31,6 +31,18 @@ abstract class StackTrace {
   factory StackTrace.fromString(String stackTraceString) = _StringStackTrace;
 
   /**
+   * Returns a representation of the current stack trace.
+   *
+   * This is similar to what can be achieved by doing:
+   *
+   *     try { throw 0; } catch (_, stack) { return stack; }
+   *
+   * The getter achieves this without throwing, except on platforms that
+   * have no other way to get a stack trace.
+   */
+  external static StackTrace get current;
+
+  /**
    * Returns a [String] representation of the stack trace.
    *
    * The string represents the full stack trace starting from