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

This reverts commit 886615d0f9.

Reason for revert:

There was an unexpected slowdown in some async benchmarks, e.g. Calls.AwaitAsyncCall.

Will revert for now and investigate next year.


Original change's description:
> [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>

TBR=kustermann@google.com,cskau@google.com

Change-Id: I5cda795cbccc01f22e0f8192473c171a4e9fca4b
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: https://github.com/dart-lang/sdk/issues/39525
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/129285
Reviewed-by: Martin Kustermann <kustermann@google.com>
Commit-Queue: Martin Kustermann <kustermann@google.com>

runtime/vm/stack_frame.h

@@ -132,12 +132,6 @@ 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) {}
@@ -157,6 +151,12 @@ 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