Fix handling of async patches.

pkg/compiler/lib/src/elements/modelx.dart

Index: pkg/compiler/lib/src/elements/modelx.dart
diff --git a/pkg/compiler/lib/src/elements/modelx.dart b/pkg/compiler/lib/src/elements/modelx.dart
index 26dab5fea1f61c9e298da7e25ebe5030f262397c..c5612a72201b3ad9e970599af6859c463fd039a1 100644
--- a/pkg/compiler/lib/src/elements/modelx.dart
+++ b/pkg/compiler/lib/src/elements/modelx.dart
@@ -341,6 +341,16 @@ class ErroneousConstructorElementX extends ErroneousElementX
   }
 
   @override
+  get _asyncMarker {
+    throw new UnsupportedError("_asyncMarker");
+  }
+
+  @override
+  set _asyncMarker(_) {
+    throw new UnsupportedError("_asyncMarker=");
+  }
+
+  @override
   get effectiveTargetInternal {
     throw new UnsupportedError("effectiveTargetInternal");
   }
@@ -2045,7 +2055,7 @@ abstract class BaseFunctionElementX extends ElementX
 
   FunctionSignature _functionSignatureCache;
 
-  AsyncMarker asyncMarker = AsyncMarker.SYNC;
+  AsyncMarker _asyncMarker = AsyncMarker.SYNC;
 
   BaseFunctionElementX(String name, ElementKind kind, Modifiers this.modifiers,
       Element enclosing)
@@ -2053,6 +2063,22 @@ abstract class BaseFunctionElementX extends ElementX
     assert(modifiers != null);
   }
 
+  AsyncMarker get asyncMarker {
+    if (isPatched) {
+      return patch.asyncMarker;
+    }
+    return _asyncMarker;
+  }
+
+  void set asyncMarker(AsyncMarker value) {
+    if (isPatched) {
+      BaseFunctionElementX function = patch;
+      function.asyncMarker = value;
+    } else {
+      _asyncMarker = value;
+    }
+  }
+
   bool get isExternal => modifiers.isExternal;
 
   bool get isInstanceMember {