[Kernel] replace function debuggable field with originalAsyncMarker field

pkg/kernel/binary.md

 This file describes the binary format of Dart Kernel.
 
 Notation
 --------
 Bitmasks are described with the syntax:
 ```scala
     Byte flags (flag1, flag2, ..., flagN)
 ```
 where 'flag<N>' is the N-th least significant bit,
 (so flag1 is the least significant bit).
@@ skipping 305 matching lines @@
 type LocalInitializer extends Initializer {
   Byte tag = 11;
   VariableDeclaration variable;
 }
 
 /*
 enum AsyncMarker {
   Sync,
   SyncStar,
   Async,
   AsyncStar

[Kevin Millikin (Google), 2017/02/22 07:59:59]

Hmm, SyncYielding is missing.

[jensj, 2017/02/22 10:36:45]

Added.

 }
 */
 
 type FunctionNode {
   // Note: there is no tag on FunctionNode.
   FileOffset fileOffset;
   FileOffset fileEndOffset;
   Byte asyncMarker; // Index into AsyncMarker above.
-  Byte debuggable; // 1 for yes, 0 for no
+  Byte originalAsyncMarker; // Index into AsyncMarker above.
   List<TypeParameter> typeParameters;
   UInt requiredParameterCount;
   List<VariableDeclaration> positionalParameters;
   List<VariableDeclaration> namedParameters;
   DartType returnType;
   Option<InferredValue> inferredReturnValue;
   Option<Statement> body;
 }
 
 type VariableReference {
@@ skipping 594 matching lines @@
 
 /* enum BaseClassKind { None, Exact, Subclass, Subtype, } */
 
 type InferredValue {
   ClassReference baseClass; // May be NullReference if kind = None.
   Byte kind; // Index into BaseClassKind.
   Byte valueBits; // See lib/type_propagation/type_propagation.dart
 }
 
 ```