Add StackTrace.fromString constructor.

tests/language/custom_await_stack_trace_test.dart

Index: tests/language/custom_await_stack_trace_test.dart
diff --git a/tests/language/custom_await_stack_trace_test.dart b/tests/language/custom_await_stack_trace_test.dart
index cff91814046c0e2ae6e56cedcd17639177852672..dfcb429bee3872b2ee0e51d7fd6df338f17cccc0 100644
--- a/tests/language/custom_await_stack_trace_test.dart
+++ b/tests/language/custom_await_stack_trace_test.dart
@@ -1,13 +1,13 @@
 import "dart:async";
 import "package:expect/expect.dart";
 
-class Blah extends StackTrace {
+class Blah implements StackTrace {
   Blah(this._trace);
-  
+
   toString() {
     return "Blah " + _trace.toString();
   }
-  
+
   var _trace;
 }