Make constructors explicit.

src/ast.h

Index: src/ast.h
diff --git a/src/ast.h b/src/ast.h
index 48fb823c41b7dad0cddbc8893603a0a1e2de382c..7f812326fae0fb6acebe987da8783ad7cb88491c 100644
--- a/src/ast.h
+++ b/src/ast.h
@@ -542,7 +542,7 @@ class ModuleLiteral: public Module {
  protected:
   template<class> friend class AstNodeFactory;
 
-  ModuleLiteral(Block* body)
+  explicit ModuleLiteral(Block* body)
       : body_(body) {
   }
 
@@ -560,7 +560,7 @@ class ModuleVariable: public Module {
  protected:
   template<class> friend class AstNodeFactory;
 
-  ModuleVariable(Variable* var)
+  explicit ModuleVariable(Variable* var)
       : var_(var) {
   }
 
@@ -599,7 +599,7 @@ class ModuleUrl: public Module {
  protected:
   template<class> friend class AstNodeFactory;
 
-  ModuleUrl(Handle<String> url) : url_(url) {
+  explicit ModuleUrl(Handle<String> url) : url_(url) {
   }
 
  private: