Reapply change that makes path-observer more agressive with property lookups.

pkg/smoke/lib/mirrors.dart

Index: pkg/smoke/lib/mirrors.dart
diff --git a/pkg/smoke/lib/mirrors.dart b/pkg/smoke/lib/mirrors.dart
index d30baa2ef90df4df998d6cbf1fedc2faf314a9aa..c49c536adcffe6dd6bb2ce6ee66739088261aced 100644
--- a/pkg/smoke/lib/mirrors.dart
+++ b/pkg/smoke/lib/mirrors.dart
@@ -218,7 +218,18 @@ Symbol _setterName(Symbol getter) =>
 
 ClassMirror _safeSuperclass(ClassMirror type) {
   try {
-    return type.superclass;
+    var t = type.superclass;
+    // TODO(sigmund): workaround for darbug.com/17779.
+    // Interceptor is leaked by dart2js. It has the same methods as Object
+    // (including noSuchMethod), and our code above assumes that it doesn't
+    // exist. Most queries exclude Object, so they should exclude Interceptor
+    // too. We don't check for t.simpleName == #Interceptor because depending on
+    // dart2js optimizations it may be #Interceptor or #num/Interceptor.
+    // Checking for a private library seems to reliably filter this out.
+    if (t != null && t.owner != null && t.owner.isPrivate) {
+      t = _objectType;
+    }
+    return t;
   } on UnsupportedError catch (e) {
     // Note: dart2js throws UnsupportedError when the type is not reflectable.
     return _objectType;