Move a definition to fix a compiler error

runtime/vm/kernel.h

Index: runtime/vm/kernel.h
diff --git a/runtime/vm/kernel.h b/runtime/vm/kernel.h
index 227a225ce9854f202454ad3eb59a13f58a5a7d82..5125d4459a235769b219d59d0311d25931bf8e72 100644
--- a/runtime/vm/kernel.h
+++ b/runtime/vm/kernel.h
@@ -935,12 +935,7 @@ class FunctionNode : public TreeNode {
   DartType* return_type() { return return_type_; }
 
   Statement* body() { return body_; }
-  void ReplaceBody(Statement* body) {
-    delete body_;
-    // Use static_cast to invoke the conversion function and so avoid triggering
-    // ASSERT(pointer_ == NULL) in operator= when overwriting a non-NULL body.
-    static_cast<Statement*&>(body_) = body;
-  }
+  void ReplaceBody(Statement* body);
 
   TokenPosition position() { return position_; }
   TokenPosition end_position() { return end_position_; }