Land fix to StackTrace.current performance already landed in dart2js.

tool/input_sdk/patch/core_patch.dart

Index: tool/input_sdk/patch/core_patch.dart
diff --git a/tool/input_sdk/patch/core_patch.dart b/tool/input_sdk/patch/core_patch.dart
index d557375c54b5bf3662b5ce40c99891fcc627bc14..52ddebdc8205f0098a60387800b92fa3c3ca3566 100644
--- a/tool/input_sdk/patch/core_patch.dart
+++ b/tool/input_sdk/patch/core_patch.dart
@@ -12,7 +12,8 @@ import 'dart:_js_helper' show patch,
                               JSSyntaxRegExp,
                               Primitives,
                               stringJoinUnchecked,
-                              objectHashCode;
+                              objectHashCode,
+                              getTraceFromException;
 
 import 'dart:_foreign_helper' show JS;
 
@@ -458,16 +459,14 @@ class StackTrace {
   @patch
   @NoInline()
   static StackTrace get current {
-    var error = JS('', 'new Error()');
-    var stack = JS('String|Null', '#.stack', error);
-    if (stack is String) return new StackTrace.fromString(stack);
     if (JS('', 'Error.captureStackTrace') != null) {
+      var error = JS('', 'new Error()');
       JS('void', 'Error.captureStackTrace(#)', error);
-      var stack = JS('String|Null', '#.stack', error);
-      if (stack is String) return new StackTrace.fromString(stack);
+      return getTraceFromException(error);
     }
+    // Fallback if Error.captureStackTrace does not exist.
     try {
-      throw 0;
+      throw '';
     } catch (_, stackTrace) {
       return stackTrace;
     }