Reapply "Improve hashCode for closure objects" with fixes.

runtime/lib/function.dart

Index: runtime/lib/function.dart
diff --git a/runtime/lib/function.dart b/runtime/lib/function.dart
index 7f0556de37369ccc5dda11d71eea2b0e25f49832..05a20a46bf3f28e47cbb5e79f7bac4238708d969 100644
--- a/runtime/lib/function.dart
+++ b/runtime/lib/function.dart
@@ -5,16 +5,23 @@
 class _Closure implements Function {
   bool operator ==(other) native "Closure_equals";
 
-  int get hashCode native "Closure_hashCode";
+  int get hashCode {
+    if (_hash == null) {
+      _hash = _computeHash();
+    }
+    return _hash;
+  }
 
   _Closure get call => this;
 
   _Closure _clone() native "Closure_clone";
 
+  int _computeHash() native "Closure_computeHash";
+
   // No instance fields should be declared before the following 4 fields whose
   // offsets must be identical in Dart and C++.
 
-  // The following 4 fields are declared both in raw_object.h (for direct access
+  // The following fields are declared both in raw_object.h (for direct access
   // from C++ code) and also here so that the offset-to-field map used by
   // deferred objects is properly initialized.
   // Caution: These fields are not Dart instances, but VM objects. Their Dart
@@ -23,4 +30,13 @@ class _Closure implements Function {
   var _function_type_arguments;
   var _function;
   var _context;
+
+  // Note: _Closure objects are created by VM "magically", without invoking
+  // constructor. So, _Closure default constructor is never compiled and
+  // detection of default-initialized fields is not performed.
+  // As a consequence, VM incorrectly assumes that _hash field is not
+  // nullable and may incorrectly remove 'if (_hash == null)' in get:hashCode.
+  // This initializer makes _hash field nullable even without constructor
+  // compilation.
+  var _hash = null;
 }