[VM runtime] Introduce a new VM class Bytecode.

Allow pc_marker slot to hold a Code object or a new Bytecode object.

Change-Id: If11c1df6dafc5b1cfcce6f0322c36d1d68e86df9
Reviewed-on: https://dart-review.googlesource.com/c/82526
Commit-Queue: Régis Crelier <regis@google.com>
Reviewed-by: Ryan Macnak <rmacnak@google.com>
Reviewed-by: Alexander Markov <alexmarkov@google.com>

runtime/vm/stack_frame.h

@@ -165,6 +165,7 @@ class StackFrame : public ValueObject {
 
   RawFunction* LookupDartFunction() const;
   RawCode* LookupDartCode() const;
+  RawBytecode* LookupDartBytecode() const;
   bool FindExceptionHandler(Thread* thread,
                             uword* handler_pc,
                             bool* needs_stacktrace,
@@ -189,7 +190,7 @@ class StackFrame : public ValueObject {
 
  private:
   RawCode* GetCodeObject() const;
-  RawCode* UncheckedGetCodeObject() const;
+  RawBytecode* GetBytecodeObject() const;
 
   uword GetCallerSp() const {
     return fp() +