Add StackTrace.fromString constructor.

tests/language/custom_await_stack_trace_test.dart

 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;
 }
 
 foo() {
   var x = "\nBloop\nBleep\n";
   return new Future.error(42, new Blah(x));
 }
 
 main() async {
   try {
     var x = await foo();
     Expect.fail("Should not reach here.");
   } on int catch (e, s) {
     Expect.equals(42, e);
     Expect.equals("Blah \nBloop\nBleep\n", s.toString());
     return;
   }
   Expect.fail("Unreachable.");
 }