Do not create empty blocks on mutations

lib/mutant/mutator/block.rb

@@ -3,7 +3,7 @@ module Mutant
     # Mutator on AST blocks
     class Block < self
 
-      handle Rubinius::AST::Block
+      handle(Rubinius::AST::Block)
 
     private
 
@@ -16,7 +16,11 @@ module Mutant
       def dispatch
         array = node.array
         emit_elements(array)
-        emit_element_presence(array)
+        if array.length > 1
+          emit_element_presence(array)
+        else
+          emit_self([new_nil])
+        end
       end
     end
   end

spec/shared/mutator_behavior.rb

@@ -3,7 +3,10 @@ shared_examples_for 'a mutator' do
 
   let(:yields)        { []              }
   let(:object)        { described_class }
-  let(:node)          { source.to_ast   }
+
+  unless instance_methods.map(&:to_s).include?('node')
+    let(:node)          { source.to_ast   }
+  end
 
   it_should_behave_like 'a command method'
 

spec/unit/mutant/mutator/block/mutation_spec.rb

@@ -1,20 +1,36 @@
 require 'spec_helper'
 
 describe Mutant::Mutator, 'block' do
-  # Two send operations
-  let(:source) { "self.foo\nself.bar" }
 
-  let(:mutations) do
-    mutations = []
+  context 'with more than one statement' do
+    let(:source) { "self.foo\nself.bar" }
 
-    # Mutation of each statement in block
-    mutations << "foo\nself.bar"
-    mutations << "self.foo\nbar"
+    let(:mutations) do
+      mutations = []
 
-   ## Remove statement in block
-    mutations << [:block, 'self.foo'.to_sexp]
-    mutations << [:block, 'self.bar'.to_sexp]
+      # Mutation of each statement in block
+      mutations << "foo\nself.bar"
+      mutations << "self.foo\nbar"
+
+     ## Remove statement in block
+      mutations << [:block, 'self.foo'.to_sexp]
+      mutations << [:block, 'self.bar'.to_sexp]
+    end
+
+    it_should_behave_like 'a mutator'
   end
 
-  it_should_behave_like 'a mutator'
+
+  context 'with one statement' do
+    let(:node) { Rubinius::AST::Block.new(1, ['self.foo'.to_ast]) }
+
+    let(:mutations) do
+      mutations = []
+      mutations << [:block, 'foo'.to_sexp]
+      # Empty blocks result in stack verification error
+      mutations << [:block, 'nil'.to_sexp]
+    end
+
+    it_should_behave_like 'a mutator'
+  end
 end