No need to check if YAML::ENGINE is defined since ruby 1.9 does that

activerecord/lib/active_record/core.rb

@@ -319,7 +319,7 @@ def inspect
 
     # Hackery to accomodate Syck. Remove for 4.0.
     def to_yaml(opts = {}) #:nodoc:
-      if YAML.const_defined?(:ENGINE) && !YAML::ENGINE.syck?
+      if !YAML::ENGINE.syck?
         super
       else
         coder = {}

activesupport/lib/active_support/core_ext/big_decimal/conversions.rb

@@ -16,7 +16,7 @@ class BigDecimal
   #
   # Note that reconstituting YAML floats to native floats may lose precision.
   def to_yaml(opts = {})
-    return super if defined?(YAML::ENGINE) && !YAML::ENGINE.syck?
+    return super if !YAML::ENGINE.syck?
 
     YAML.quick_emit(nil, opts) do |out|
       string = to_s

activesupport/lib/active_support/core_ext/string/output_safety.rb

@@ -137,7 +137,7 @@ def encode_with(coder)
     end
 
     def to_yaml(*args)
-      return super() if defined?(YAML::ENGINE) && !YAML::ENGINE.syck?
+      return super() if !YAML::ENGINE.syck?
       to_str.to_yaml(*args)
     end
 

activesupport/lib/active_support/ordered_hash.rb

@@ -30,7 +30,7 @@ def encode_with(coder)
     end
 
     def to_yaml(opts = {})
-      if YAML.const_defined?(:ENGINE) && !YAML::ENGINE.syck?
+      if !YAML::ENGINE.syck?
         return super
       end
 

activesupport/lib/active_support/time_with_zone.rb

@@ -147,7 +147,7 @@ def encode_with(coder)
     end
 
     def to_yaml(options = {})
-      return super if defined?(YAML::ENGINE) && !YAML::ENGINE.syck?
+      return super if !YAML::ENGINE.syck?
 
       utc.to_yaml(options)
     end