Remove uses of String.operator+.

utils/template/world.dart

Index: utils/template/world.dart
diff --git a/utils/template/world.dart b/utils/template/world.dart
index 0d0369904de66aae869190bd2ab8a91fde651b48..333e67bf9b13e8adc5b21430eef8f63c1886b275 100644
--- a/utils/template/world.dart
+++ b/utils/template/world.dart
@@ -72,7 +72,7 @@ class World {
       }
     } else {
       final messageWithPrefix = options.useColors
-          ? (color + prefix + _NO_COLOR + message) : (prefix + message);
+          ? "$color$prefix$_NO_COLOR$message" : "$prefix$message";
 
       var text = messageWithPrefix;
       if (span != null) {
@@ -98,7 +98,7 @@ class World {
     }
 
     if (throwing) {
-      throw new CompilerException(prefix + message, span);
+      throw new CompilerException("$prefix$message", span);
     }
   }