Restrict yield outside generators in strict mode

* Move some yield/generic tests from compress/harmony.js to
  compress/yield.js
 * Adjust error messages to conform ecmascript standards

test/mocha/class.js

@@ -16,11 +16,11 @@ describe("Class", function() {
         }
         var error = function(e) {
             return e instanceof uglify.JS_Parse_Error &&
-                e.message === "Unexpected token: expand (...)";
+                e.message === "SyntaxError: Unexpected token: expand (...)";
         }
 
         for (var i = 0; i < tests.length; i++) {
             assert.throws(test(tests[i]), error);
         }
     });
-});
+});