Display the awaiter call stack in Observatory

runtime/observatory/lib/src/elements/debugger.dart

Index: runtime/observatory/lib/src/elements/debugger.dart
diff --git a/runtime/observatory/lib/src/elements/debugger.dart b/runtime/observatory/lib/src/elements/debugger.dart
index 9b1cc616dcdc8b3aae625a892dad196cefd41f3a..c4b445677608b4e019297ef41978152468a9c562 100644
--- a/runtime/observatory/lib/src/elements/debugger.dart
+++ b/runtime/observatory/lib/src/elements/debugger.dart
@@ -600,9 +600,14 @@ class SetCommand extends DebuggerCommand {
     ],
     'causal-async-stacks': [
       _boolValues,
-      _setSaneAsyncStacks,
+      _setCausalAsyncStacks,
       (debugger, _) => debugger.saneAsyncStacks
     ],
+    'awaiter-stacks': [
+      _boolValues,
+      _setAwaiterStacks,
+      (debugger, _) => debugger.awaiterStacks
+    ]
   };
 
   static Future _setBreakOnException(debugger, name, value) async {
@@ -624,16 +629,22 @@ class SetCommand extends DebuggerCommand {
     debugger.console.print('${name} = ${value}');
   }
 
-  static Future _setSaneAsyncStacks(debugger, name, value) async {
+  static Future _setCausalAsyncStacks(debugger, name, value) async {
     if (value == 'true') {
-      debugger.saneAsyncStacks = true;
+      debugger.causalAsyncStacks = true;
     } else {
-      debugger.saneAsyncStacks = false;
+      debugger.causalAsyncStacks = false;
     }
     debugger.refreshStack();
     debugger.console.print('${name} = ${value}');
   }
 
+  static Future _setAwaiterStacks(debugger, name, value) async {
+    debugger.awaiterStacks = (value == 'true');
+    debugger.refreshStack();
+    debugger.console.print('${name} == ${value}');
+  }
+
   Future run(List<String> args) async {
     if (args.length == 0) {
       for (var name in _options.keys) {
@@ -1363,14 +1374,23 @@ class ObservatoryDebugger extends Debugger {
 
   bool _upIsDown;
 
-  bool get saneAsyncStacks => _saneAsyncStacks;
-  void set saneAsyncStacks(bool value) {
+  bool get causalAsyncStacks => _causalAsyncStacks;
+  void set causalAsyncStacks(bool value) {
     settings.set('causal-async-stacks', value);
-    _saneAsyncStacks = value;
+    _causalAsyncStacks = value;
+  }
+
+  bool _causalAsyncStacks;
+
+  bool get awaiterStacks => _awaiterStacks;
+  void set awaiterStacks(bool value) {
+    settings.set('awaiter-stacks', value);
+    _causalAsyncStacks = value;
   }
 
-  bool _saneAsyncStacks;
+  bool _awaiterStacks;
 
+  static const String kAwaiterStackFrames = 'awaiterFrames';
   static const String kAsyncCausalStackFrames = 'asyncCausalFrames';
   static const String kStackFrames = 'frames';
 
@@ -1391,31 +1411,35 @@ class ObservatoryDebugger extends Debugger {
   }
 
   int get stackDepth {
-    if (saneAsyncStacks) {
+    if (awaiterStacks) {
+      var awaiterStackFrames = stack[kAwaiterStackFrames];
+      if (awaiterStackFrames != null) {
+        return awaiterStackFrames.length;
+      }
+    }
+    if (causalAsyncStacks) {
       var asyncCausalStackFrames = stack[kAsyncCausalStackFrames];
-      var stackFrames = stack[kStackFrames];
-      if (asyncCausalStackFrames == null) {
-        // No causal frames.
-        return stackFrames.length;
+      if (asyncCausalStackFrames != null) {
+        return asyncCausalStackFrames.length;
       }
-      return asyncCausalStackFrames.length;
-    } else {
-      return stack[kStackFrames].length;
     }
+    return stack[kStackFrames].length;
   }
 
   List get stackFrames {
-    if (saneAsyncStacks) {
+    if (awaiterStacks) {
+      var awaiterStackFrames = stack[kAwaiterStackFrames];
+      if (awaiterStackFrames != null) {
+        return awaiterStackFrames;
+      }
+    }
+    if (causalAsyncStacks) {
       var asyncCausalStackFrames = stack[kAsyncCausalStackFrames];
-      var stackFrames = stack[kStackFrames];
-      if (asyncCausalStackFrames == null) {
-        // No causal frames.
-        return stackFrames ?? [];
+      if (asyncCausalStackFrames != null) {
+        return asyncCausalStackFrames;
       }
-      return asyncCausalStackFrames;
-    } else {
-      return stack[kStackFrames] ?? [];
     }
+    return stack[kStackFrames] ?? [];
   }
 
   static final _history = [''];
@@ -1453,7 +1477,8 @@ class ObservatoryDebugger extends Debugger {
 
   void _loadSettings() {
     _upIsDown = settings.get('up-is-down');
-    _saneAsyncStacks = settings.get('causal-async-stacks') ?? true;
+    _causalAsyncStacks = settings.get('causal-async-stacks') ?? true;
+    _awaiterStacks = settings.get('awaiter-stacks') ?? true;
   }
 
   S.VM get vm => page.app.vm;
@@ -2272,7 +2297,10 @@ class DebuggerStackElement extends HtmlElement implements Renderable {
   void updateStackFrames(S.ServiceMap newStack) {
     List frameElements = _frameList.children;
     List newFrames;
-    if (_debugger.saneAsyncStacks &&
+    if (_debugger.awaiterStacks &&
+        (newStack[ObservatoryDebugger.kAwaiterStackFrames] != null)) {
+      newFrames = newStack[ObservatoryDebugger.kAwaiterStackFrames];
+    } else if (_debugger.causalAsyncStacks &&
         (newStack[ObservatoryDebugger.kAsyncCausalStackFrames] != null)) {
       newFrames = newStack[ObservatoryDebugger.kAsyncCausalStackFrames];
     } else {