Generalize all error messages with "experimental in alpha release" to

just say "experimental."

src/libsyntax/feature_gate.rs

@@ -430,7 +430,7 @@ impl<'a, 'v> Visitor<'v> for PostExpansionVisitor<'a> {
             ast::ExprBox(..) | ast::ExprUnary(ast::UnOp::UnUniq, _) => {
                 self.gate_feature("box_syntax",
                                   e.span,
-                                  "box expression syntax is experimental in alpha release; \
+                                  "box expression syntax is experimental; \
                                    you can call `Box::new` instead.");
             }
             ast::ExprLit(ref lit) => {

src/test/compile-fail/feature-gate-box-expr.rs

@@ -11,13 +11,13 @@
 fn main() {
     use std::boxed::HEAP;
 
-    let x = box 'c'; //~ ERROR box expression syntax is experimental in alpha release
+    let x = box 'c'; //~ ERROR box expression syntax is experimental
     println!("x: {}", x);
 
-    let x = box () 'c'; //~ ERROR box expression syntax is experimental in alpha release
+    let x = box () 'c'; //~ ERROR box expression syntax is experimental
     println!("x: {}", x);
 
-    let x = box (HEAP) 'c'; //~ ERROR box expression syntax is experimental in alpha release
+    let x = box (HEAP) 'c'; //~ ERROR box expression syntax is experimental
     println!("x: {}", x);
 }
 

src/test/compile-fail/feature-gate-box-pat.rs

@@ -9,6 +9,6 @@
 // except according to those terms.
 
 fn main() {
-    let box x = Box::new('c'); //~ ERROR box pattern syntax is experimental in alpha release
+    let box x = Box::new('c'); //~ ERROR box pattern syntax is experimental
     println!("x: {}", x);
 }