fix spelling in core libraries

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 17abd3831ed957102d61eaa16093b9e31e33a8a4..e655072b10d93cb6dd37008b3ff6758fd2981e94 100644
--- a/sdk/lib/core/stacktrace.dart
+++ b/sdk/lib/core/stacktrace.dart
@@ -46,7 +46,7 @@ abstract class StackTrace {
    * Returns a [String] representation of the stack trace.
    *
    * The string represents the full stack trace starting from
-   * the point where a throw ocurred to the top of the current call sequence.
+   * the point where a throw occurred to the top of the current call sequence.
    *
    * The exact format of the string representation is not final.
    */