Merge branch 'optimize_concat' of https://github.com/rvanvelzen/UglifyJS2 into rvanve...

diff --cc lib/compress.js
index dcd376c,fce0757..ed664a2
--- 1/lib/compress.js
--- 2/lib/compress.js
+++ b/lib/compress.js
@@@ -1656,6 -1642,11 +1656,11 @@@ merge(Compressor.prototype, 
                case "<=": reverse(">="); break;
              }
          }
 -            && self.left.operator == "+" && self.left.left instanceof AST_String) {
+         if (self.operator == "+" && self.right instanceof AST_String
+             && self.right.getValue() === "" && self.left instanceof AST_Binary
++            && self.left.operator == "+" && self.left.is_string()) {
+             return self.left;
+         }
          return self;
      });