Migrate to interpolation instead of concatenation.

lib/json/json.dart

 // Copyright (c) 2011, 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.
 
 #library("json");
 
 // Pure Dart implementation of JSON protocol.
 
 /**
  * Utility class to parse JSON and serialize objects to JSON.
@@ skipping 125 matching lines @@
   static final int E_SMALL = 101;  // 'e'.charCodeAt(0)
   static final int F_SMALL = 102;  // 'f'.charCodeAt(0)
   static final int N_SMALL = 110;  // 'n'.charCodeAt(0)
   static final int R_SMALL = 114;  // 'r'.charCodeAt(0)
   static final int T_SMALL = 116;  // 't'.charCodeAt(0)
   static final int U_SMALL = 117;  // 'u'.charCodeAt(0)
   static final int Z_SMALL = 122;  // 'z'.charCodeAt(0)
   static final int LBRACE = 123;  // '{'.charCodeAt(0)
   static final int RBRACE = 125;  // '}'.charCodeAt(0)
 
-  JsonTokenizer(String s) : _s = s + ' ', _pos = 0, _len = s.length + 1 {}
+  JsonTokenizer(String s) : _s = '${s} ', _pos = 0, _len = s.length + 1;
 
   /**
    * Fetches next token or [:null:] if the stream has been exhausted.
    */
   JsonToken next() {
     while (_pos < _len && isWhitespace(_s.charCodeAt(_pos))) {
       _pos++;
     }
     if (_pos == _len) {
       return null;
@@ skipping 35 matching lines @@
                 c = CARRIAGE_RETURN;
                 break;
               case 'f':
                 c = FORM_FEED;
                 break;
               case 't':
                 c = TAB;
                 break;
               case 'u':
                 if (_pos + 5 > _len) {
-                  throw 'Invalid unicode esacape sequence: \\' +
-                      _s.substring(_pos, _len);
+                  throw 'Invalid unicode esacape sequence:'
+                      '\\${_s.substring(_pos, _len)}';
                 }
                 final codeString = _s.substring(_pos + 1, _pos + 5);
-                c = Math.parseInt('0x' + codeString);
+                c = Math.parseInt('0x${codeString}');
                 if (c >= 128) {
                   // TODO(jmessery): the VM doesn't support 2-byte strings yet
                   // see runtime/lib/string.cc:49
                   // So instead we replace these characters with '?'
                   c = '?'.charCodeAt(0);
                 }
                 _pos += 4;
                 break;
               default:
-                throw 'Invalid esacape sequence: \\' + _s[_pos];
+                throw 'Invalid esacape sequence: \\${_s[_pos]}';
             }
           }
           charCodes.add(c);
           _pos++;
         }
         if (_pos == _len) {
           throw 'Unmatched quote';
         }
 
         final String body = new String.fromCharCodes(charCodes);
@@ skipping 386 matching lines @@
         });
         _sb.add('}');
         _seen.removeLast();
         return;
 
       default:
         throw const JsonUnsupportedObjectType();
     }
   }
 }