Tolerate "isset($c->prop)" for collections

Currently collections define magic __get, __set, __isset, and __unset
methods which will throw exceptions if any property access is attempted
on collections (such as "isset($vec->prop)" and "$x = $vec->prop").

However, there is existing code that uses isset() to check if various
objects have a property with a certain name, and it feels like throwing
an exception here is a little harsh. This diff updates collections to
be tolerant of reading properties.

hphp/runtime/base/builtin_functions.cpp

@@ -453,7 +453,7 @@ void throw_collection_modified() {
 
 void throw_collection_property_exception() {
   Object e(SystemLib::AllocInvalidOperationExceptionObject(
-    "Cannot access property on a collection"));
+    "Cannot access a property on a collection"));
   throw e;
 }
 

hphp/runtime/ext/ext_collections.cpp

@@ -4567,7 +4567,7 @@ void c_PairIterator::t_rewind() {
     throw_collection_property_exception(); \
   } \
   bool c_##cls::t___isset(Variant name) { \
-    throw_collection_property_exception(); \
+    return false; \
   } \
   Variant c_##cls::t___unset(Variant name) { \
     throw_collection_property_exception(); \

hphp/test/slow/collection_classes/805.php

@@ -8,14 +8,25 @@ try {
     $obj = new $cls;
     try {
       $x = $obj->foo;
+      echo "get: ";
+      var_dump($x);
     } catch (RuntimeException $e) {
-      echo $i;
+      echo "get throws, i = ";
+      var_dump($i);
+    }
+    try {
+      $x = isset($obj->foo);
+      echo "isset: ";
+      var_dump($x);
+    } catch (RuntimeException $e) {
+      echo "isset throws, i = ";
+      var_dump($i);
     }
-    ++$i;
     try {
       $obj->foo = 123;
     } catch (RuntimeException $e) {
-      echo $i;
+      echo "set throws, i = ";
+      var_dump($i);
     }
     ++$i;
   }

hphp/test/slow/collection_classes/805.php.expect

@@ -1 +1,10 @@
-012345Done
+get throws, i = int(0)
+isset: bool(false)
+set throws, i = int(0)
+get throws, i = int(1)
+isset: bool(false)
+set throws, i = int(1)
+get throws, i = int(2)
+isset: bool(false)
+set throws, i = int(2)
+Done