Add support for implicitly concatenating adjacent string literals.

frog/gen.dart

 // Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 /**
  * Top level generator object for writing code and keeping track of
  * dependencies.
  *
  * Should have two compilation models, but only one implemented so far.
  *
@@ skipping 2278 matching lines @@
 
   _isUnaryIncrement(Expression item) {
     if (item is UnaryExpression) {
       UnaryExpression u = item;
       return u.op.kind == TokenKind.INCR || u.op.kind == TokenKind.DECR;
     } else {
       return false;
     }
   }
 
+  visitStringConcatExpression(StringConcatExpression node) {
+    var items = [];
+    for (var item in node.strings) {
+      Value val = visitValue(item);
+      assert(val.type.isString);
+      items.add(val.code);
+    }
+    return new Value(world.stringType, '(${Strings.join(items, " + ")})',
+                     node.span);
+  }
+
   visitStringInterpExpression(StringInterpExpression node) {
     var items = [];
     for (var item in node.pieces) {
       var val = visitValue(item);
       val.invoke(this, 'toString', item, Arguments.EMPTY);
       // TODO(jimhug): Ensure this solves all precedence problems.
       // TODO(jmesserly): We could be smarter about prefix/postfix, but we'd
       // need to know if it will compile to a ++ or to some sort of += form.
       var code = val.code;
       if (_expressionNeedsParens(item)) {
@@ skipping 140 matching lines @@
     return true;
   }
 
 }
 
 class ReturnKind {
   static final int IGNORE = 1;
   static final int POST = 2;
   static final int PRE = 3;
 }