[SDK] Switch to is_sync to identify sync/async running.

This should address the regression introduced by https://dart-review.googlesource.com/c/sdk/+/124988

Bug: https://github.com/dart-lang/sdk/issues/39525
Change-Id: Id163b649bdd0363297c186559fa84ff87f908e4b
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/129062
Reviewed-by: Clement Skau <cskau@google.com>
Reviewed-by: Martin Kustermann <kustermann@google.com>
Commit-Queue: Clement Skau <cskau@google.com>

runtime/vm/stack_frame.h

@@ -132,6 +132,12 @@ class StackFrame : public ValueObject {
 
   static void DumpCurrentTrace();
 
+  uword GetCallerSp() const {
+    return fp() +
+           ((is_interpreted() ? kKBCCallerSpSlotFromFp : kCallerSpSlotFromFp) *
+            kWordSize);
+  }
+
  protected:
   explicit StackFrame(Thread* thread)
       : fp_(0), sp_(0), pc_(0), thread_(thread), is_interpreted_(false) {}
@@ -151,12 +157,6 @@ class StackFrame : public ValueObject {
   RawCode* GetCodeObject() const;
   RawBytecode* GetBytecodeObject() const;
 
-  uword GetCallerSp() const {
-    return fp() +
-           ((is_interpreted() ? kKBCCallerSpSlotFromFp : kCallerSpSlotFromFp) *
-            kWordSize);
-  }
-
   uword GetCallerFp() const {
     return *(reinterpret_cast<uword*>(
         fp() + ((is_interpreted() ? kKBCSavedCallerFpSlotFromFp