Remove uses of string + from API document generator.

lib/dartdoc/frog/library.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.
 
 class LibraryImport {
   final String prefix;
   final Library library;
   final SourceSpan span;
   LibraryImport(this.library, [this.prefix, this.span]);
 }
@@ skipping 197 matching lines @@
         world.warning(message, node.span);
         return world.varType;
       }
     }
     return ret;
   }
 
   static String _getDottedName(NameTypeReference type) {
     if (type.names != null) {
       var names = map(type.names, (n) => n.name);
-      return type.name.name + '.' + Strings.join(names, '.');
+      return '${type.name.name}.${Strings.join(names, ".")}';
     } else {
       return type.name.name;
     }
   }
 
   Member lookup(String name, SourceSpan span) {
     return _topNames[name];
   }
 
   resolve() {
@@ skipping 152 matching lines @@
         break;
 
       case "import":
         seenImport = true;
         name = getFirstStringArg(node);
         var prefix = tryGetNamedStringArg(node, 'prefix');
         if (node.arguments.length > 2 ||
             node.arguments.length == 2 && prefix == null) {
           world.error(
               'expected at most one "name" argument and one optional "prefix"'
-              + ' but found ${node.arguments.length}', node.span);
+              ' but found ${node.arguments.length}', node.span);
         } else if (prefix != null && prefix.indexOf('.') >= 0) {
           world.error('library prefix canot contain "."', node.span);
         } else if (seenSource || seenResource) {
           world.error('#imports must come before any #source or #resource',
             node.span);
         }
 
         // Empty prefix and no prefix are equivalent
         if (prefix == '') prefix = null;
 
@@ skipping 59 matching lines @@
 
   String tryGetNamedStringArg(DirectiveDefinition node, String argName) {
     var args = node.arguments.filter(
         (a) => a.label != null && a.label.name == argName);
 
     if (args.length == 0) {
       return null;
     }
     if (args.length > 1) {
       world.error('expected at most one "${argName}" argument but found '
-          + node.arguments.length, node.span);
+                  '${node.arguments.length}', node.span);
     }
     // Even though the collection has one arg, this is the easiest way to get
     // the first item.
     for (var arg in args) {
       return _parseStringArgument(arg);
     }
   }
 
   String _parseStringArgument(ArgumentNode arg) {
     var expr = arg.value;
@@ skipping 18 matching lines @@
 
   void visitFunctionDefinition(FunctionDefinition node) {
     currentType.addMethod(node.name.name, node);
   }
 
   void visitFunctionTypeDefinition(FunctionTypeDefinition node) {
     var type = library.addType(node.func.name.name, node, false);
     type.addMethod(':call', node.func);
   }
 }