[Kernel] replace function debuggable field with originalAsyncMarker field

pkg/kernel/lib/ast.dart

Index: pkg/kernel/lib/ast.dart
diff --git a/pkg/kernel/lib/ast.dart b/pkg/kernel/lib/ast.dart
index 2f3cb111d74337e809f435bacbdfa2000ae8f9db..e2010ccb9bfa598aed3dedd0fb794cbfaee91e6e 100644
--- a/pkg/kernel/lib/ast.dart
+++ b/pkg/kernel/lib/ast.dart
@@ -1150,8 +1150,18 @@ class FunctionNode extends TreeNode {
   /// (this is the default if none is specifically set).
   int fileEndOffset = TreeNode.noOffset;
 
+  /// Current async marker for the function.
   AsyncMarker asyncMarker;
-  bool debuggable = true;
+
+  /// Original async marker for the function.
+  ///
+  /// In source code, a function can for instance be marked as async.

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

I wouldn't discuss all these imperative details (specifically, one might just
build a new function and this discussion of updating fields doesn't make any
sense).  Instead of "current" and "original" I would adopt naming something like
dartAsyncMarker and kernelAsyncMarker and then make the comment declarative:

"A Kernel function can represent a Dart function with a different async marker."

If we need an example we can write: "For example, when async/await is translated
away, a Dart async function might be represented by a Kernel sync function."

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

Done.

+  /// This will make the async marker be [Async]. A kernel transformation can
+  /// however transform this code at mark it as [Sync]. Such a transformation
+  /// should update this field to indicate the original async status.
+  /// This way, for instance, the VM, can still behave slightly different
+  /// (e.g. make stacktraces more readable).
+  AsyncMarker originalAsyncMarker;
   List<TypeParameter> typeParameters;
   int requiredParameterCount;
   List<VariableDeclaration> positionalParameters;
@@ -1167,7 +1177,8 @@ class FunctionNode extends TreeNode {
       int requiredParameterCount,
       this.returnType: const DynamicType(),
       this.inferredReturnValue,
-      this.asyncMarker: AsyncMarker.Sync})
+      this.asyncMarker: AsyncMarker.Sync,
+      this.originalAsyncMarker: AsyncMarker.Sync})
       : this.positionalParameters =
             positionalParameters ?? <VariableDeclaration>[],
         this.requiredParameterCount =